Based on the handling of certain creatures in this season of Encounters (ending today), one of the things most likely to change by the time 5e rolls out is the grab mechanic. No surprise there, since related mechanics have been a pain in every edition in this game. Still, a standard action that merely immobilizes an opponent until it can make a move action to escape (automatically, in some cases) isn't that useful even to creatures that get damage bonuses against a target they have grabbed. Maybe this would be different if there was some option for moving the targets status from grabbed to restrained and then to helpless, but there's some reluctance to let player characters face that kind of risk.
Meanwhile, the Lost Crown of Neverwinter solution gave monsters that just grabbed their opponents with a successful attack a flat 10 hp of damage on top of this result. Maybe it would have been better to apply the same guidelines for damage done by at-will attacks that target multiple opponents, as that flat 10 was a bit harsh to some 1st level groups.
Ongoing damage needs some fixing, especially with regard to creatures that do nothing but ongoing damage. Consider the fire elemental: once one has scored a hit against a target, no other fire elemental can do anything more to that opponent, whether during their own turns or on an opportunity attack. To make it worse, tier-appropriate fire resistance makes you immune to a fire elemental of the same tier. Okay, it can still destroy the furniture, but that's likely to feel either contrived or spiteful in most cases.
I'd rather see ongoing damage replaced by a counter that lasts until the encounter ends. (I'll discuss some options for removing it earlier in a later paragraph.) Instead of causing ongoing 5 fire damage, an elemental causes level appropriate fire damage to its target, and all further fire damage during that encounter causes x amount more damage (to creatures only). Each time the elemental (or some other opponent that currently does ongoing fire damage) hits the target, the counter increases by 1 hp of damage, or maybe 1 hp per tier. This won't impair player characters too badly in most cases, but it has something to offer the game as a tool of attrition.
Removing this marker early could be done with a successful Heal check, a good use of a standard action when attrition damage can eventually wear down any resistance or regeneration you might have. You might also allow someone to remove the counter with the use of second wind, or possibly with a successful Endurance check at the cost of a healing surge.
Something similar to that last idea might serve to replace the current mechanics for saving throws. Rather than the base 55% chance of success, give creatures the option to attempt an appropriate ability check to negate an effect, losing a healing surge if successful. (Skill checks might be appropriate in some cases, but making sense of that will require more effort.) Since monsters don't have more than 1 surge per tier, multiple successes will cost them a quarter of their hit points each time. I'd suggest that it only cost elites an eighth (or maybe 10%) of their hp in such cases, while solos would only lose 5% of their total each time this happened.
As suggested, these are not workable systems. The rules would have to be significantly overhauled to make them work, particularly with regard to effects that prevent creatures from taking any effective action. That said, designers have cautioned against the use of such debilitating powers before. They mainly had their use against player characters in mind, but they can be real fun killers against monsters as well. (It's fine to stack the deck in your favour right up until you may as well be playing solitaire for all your opponents get to contribute.)
That's enough for now. I've got a few thoughts on tactical teleportation (and why I think it makes sense to be more widely available than reliable flight to most creatures), and maybe a few suggestions on the handling of the ability scores (concerning with application to attack rolls), but I'll save those for a later post.