AnotherGuy
Hero
In 3.5e the psion used points and the psionic powers were beautifully crafted in that they allowed a lot of flexibility.Spell slots, unless you MUST specifically prepare every spellslot (which no longer exists in D&D), absolutely does not force more variety compared to mana. You can do the same spell over and over with spellslots, using higher level slots to upcast it. DC20's "upcast" system is far more sophisticated than D&D's, and using mana as the system plays into facilitating a wider amount of variety, since there's more possibility of changing your spells depending on how much mana you want to spend.
A little of this was carried through to the 5e's sorcerer with his sorcery points and metamagic capabilities.
However what appears to be missing within your analysis of 5e is that the engine used by a 5e casting class enforces a particular theme.
- Wizards are bookish and thus their spells are structured and perfectly prepared with little room for flexibility. (We can certainly debate whether there should be more upcast capability or ritual casting); while
- Sorcerers are suited to a mana pool which they expend to twist and manipulate spells in weird and wonderful ways.
If everyone uses the same system - you have sacrificed the theme for flexibility.
Unfortunately in DC20 it appears your "cheesing your bonus action" has been replaced by "cheesing your action points".Wrong. DC20 has penalties for trying to do certain things over and over, and the flexibility of the action economy already promotes more variety on its own. D&D does not encourage variety. It encourages building around cheesing your bonus action as much as you can.
It's not a fix but a replacement by those that don't enjoy the D&D method. Nothing was solved, instead you allowed for power gamers to power game more.
Very few things which I wanted addressed were addressed by DC20 and what I instead found was that DC20 created new unnecessary issues.