Ok, so what is it then? Design the psionic system that WotC should use and please post it here.
If possible, writeup or send an example of the kind of mechanics that you have in mind. It is ok if from an earlier edition, enough to get a sense of it. Then I can give a more helpful critique.
I will mainly scrutinize it for balance for the context of 5e, but flavor including how mechanics actualize flavor is also important.
I'll post the basics for you, but be cognizant of what you're attempting to do here: You're going to attempt to critique a system used for 20 years, that was enjoyed by many players, based upon a high level overview. I admire your confidence in your abilities, but have extreme doubts as to the utility of the exercise. Putting aside that scrutinizing for balance without seeing an entire system is kind of a pointless exercise, here are the basics of the system I used from 3E that I still, currrently, have been working in 'behind the scenes' for NPCs and monsters in 5E (and for a couple one shot PCs).
1.) Psionic PCs gain a number of Defenses/Attacks. Most of the classes get to pick the ratio. For example, a first level Psion would get 4, which could be 4 defenses, 4 attacks, or a combination that mixes them. A high level Psion is the only class that historically could get all 10. They also gain powers, which are separate from attacks/defenses, although not entirely separate as discussed below. They also gain a number of psionic power points to power their abilities - the mechanics for how many PPs were obtained and how many each attack/defense/power cost changed to adapt to the editions.
2.) Each defense grants a minor defensive ability when up. While there is a tendency for these to be helpful against psionic abilities, they have broader uses. There is a cost to invoke them, and a cost to maintain them (although there are class mechanics that reduce these costs). You can have as many defenses up at a time as you want (up to the maximum of the 5 that exist). These changed over the editions to adjust to the current rule set. At low levels, you'd need to use your entire action to set up one defense. Abilities gained as you leveled up would allow for more efficienct ways to set up defenses. In 5E terminology, you might get the ability to put up a defense as a reaction, as a bonus action, or the ability to initiate multiple defenses as an action.
3.) Each attack has an effect when initiated, and then a lingering effect. You have to pay a cost to keep that lingering effect going, but similar to psionic defenses, there are mechanics to reduce these costs. Over the edditions the effects changed to accomodate the current rule set. However, the tendency was that only one dealt damage directly, while others did things like move a target, restrict their movement, made them more vulnerable to certain damage, etc... The real kicker of the lingering effect was that a target subject to one of these lingering effects would also be suscpetible to additional seconday effects for psionic powers. Like defenses, as you advance, you'd obtain more efficient ways to invoke psionic attacks allowing for multiple to be unloaded at a time on a target, or for multiple targets to be hit at once.
4.) The 5 defenses and 5 offenses have a 'rock-paper-scissors' style interaction. Each attack has one defense, that if active on the target, strongly reduces the initial attack effect and stops the lingering effect. It also has two defenses, that if active on the target, stop the lingering effect (but have no impact on the initial effect). So, if you have 2 psionic defenses that are not 'adjacent' and both are up, you'd seem to be protected from lingering effects of psionic attacks (but see below).
5.) The baseline for balance is that a target has no psionic defenses. This means that a psionic character with defenses has greater ability to defend against psionic attack, generally speaking. Much like wizards are better at fighting wizards than a fighter is, especially historically, a psion is better at defending against a psion.
6.) However, when a psionic attack targets a character with a psionic defense that reduces the effectiveness of the psionic attack, there is a way for the psionic attack to 'break' the defense. The rules for this changed over the years, with simple dice rolls used at times, and with a more complex card based system used at others (designed by a player - very cool and iconic, but too time consuming). Regardless, the way this usually played out was that when psionic PCs fought each other, they would accelerate their power point usage to break defenses and restore their defenses extra times.
7.) For each power that a psionic character has (which is different than a psionic attack or psionic defense), there is a primary ability, and then secondary abilities that can be added
if there is a creature currently subject to a lingering effect within range.
8.) Each psionic classes had class abilities that were not tied to attacks, defenses and powers as well.
9.) You had a maximum number of psionic power points. When you rested, you recovered half of what you were down, with a minimum number that you'd recover. This meant that if you depleted yourself entirely, it could take a while before you fully recovered (days), while if you kept yourself from depleting too many, you'd stay at full strength. The way this played out is that PCs tried to avoid going too deep into their reserves - but when the big finale came up they could dig into those deep reserves and go for it.
Those are the basics, but there is a lot more to it.
Here is an example of how it all plays out together: Dryxus the Psion is standing in the back of the party as their sneaky barbarian opens the door and reveals a Psionic Illithid (built using these rules) and several thralls. The other PCs engage the thralls and Dryxus engages the Illithid. Dryxus goes before the Illithid in initiative. Before the door was opened, Dryxus had established two psionic defenses (Intellect Fortress, which is best against Mind Blast and assists versus Id Insinuation and Psychic Crush), and Mental Barrier (which is best against Ego Whip and assists versus Psychic Crush and Mind Thrust).
Dryxus gets to go before the Illithid. As a Psion, Dryxus has a probe ability that allows him to spend a PP to see what psionic defense a target has up. He uses that ability and discovers that the Illithis has an Intellect Fortress protecting it, but no other defenses up. This means that he is most vulnerable to the Ego Whip and Mind Thrust attacks, as the Intellect Fortress provides no benefit against them - leaving the Illithid as vulnerable as a non-psion to those attacks, and allowing him to use powers with riders based up lingering effects from those attacks to work. He uses an action to do a dual psionic attack, launching both the Ego Whip and Mind Thrust at the target. There are two saving throws, both failed. The Illithid is knocked prone by the Ego Whip (initial effect) and pushed 5 feet by the Mind Thrust (initial effect). Further, Dryxus pays to keep the lingering effects going for both attacks, resulting in a limited vulnerability to psychic damage from the Ego Whip and a 'start of turn' chance (likely a saving throw in 5E) to be pushed back by the Mind Thrust.
The Illithid goes next. The Illithid has a series of options here to respond. They might add a psychic defense such as Mental Barrier or Thought Shield to protect themselves from the lingering effect of the Ego Whip and Mind Thrust. Or, they might go on the offensive and focus on an attack as spending all their time on defense and nothing on offense may mean they can try to outlast the Psion, but it wastes their power and there are other threats in the room as well. They have an ability that allows them to use Mind Blast whenever they us a Psionic Power. They do not know that Dryxus is prepared with the best defense against a Mind Blast, so they target Dryxus with the Mind Blast as part of using their 'Swallow Will' power. Because Dryxus has that Intellect Fortress Up, the Mind last will not impact him this turn, but there is a possibility that the Intellect Fortress will be broken by the Mind Blast and be dropped. As mentioned above, we've used a variety of mechanics for determining how this works, and in 5E I would likely have the mechanics tied to monster design / class design. Let's say that it breaks the Intellect Fortress (ending that defense for Dryxus). However, when the Swallow Will power is used on Dryxus, he would not be subject to the 'Mind Blast' secondary benefit listed in the power because there is no Mind Blast lingering effect on him.
Powers had some similarities to spells, but there were distinct differences between spells and powers. The 'tiers' of effect they might have based upon which lingering effects derived from the psionic attacks that were in play were one significant difference. Another was that they would be customized based upon your PP expenditure. For example, a single target power could be augmented to include additional targets, a greater range, higher damage, etc...
Psionic characters had far fewer 'powers' than spellcasting classes had spells (historically - the reduction in spells available in 5E would impact how this was approached). Psions had the most powers, with psychic warriors having far fewer (and fewer PPs). The idea here was to emulate the feel of a super hero with a smaller breadth of a tool set than a wizard might have - but greater use cases for each tool they had.
Power designs originated from the materials from WotC/TSR, but they evolved away from the books so that by the time we approached the end of 3.5, my system was essentially entirely homebrew, but you could see where elements originated in the rules.
How easy was it to follow and understand these rules? It was a step up from your basic wizard, but players understood it pretty quickly and thoroughly, especially when they played a psionic PC. If you were not playing a psionic PC, you didn't need to understand the inner workings... but most players picked it up as they watch the psionic PCs.
On top of all of this were some of the historical 'meta' rules of psionics that applied across the board. For example, they were not magic, so detect magic/dispel magic had no impact. However, wizards did develop magics to detect psionics and fight psionic abilities. They were uncommon, as psionics were very uncommon themselves, but they did exist.
There was more to it - we did not discuss hwo sciences and disciplines impacted the availability of powers to learn, for example, but that is the core of it.
