I would love to see more ways to specialise your character in a certain direction, and would love to have the perks turn into more of a semi skill-tree situation, where more are dependent on previous perks in order to be selected. The TES setting is very open-ended and so far I have only been able to discern rather limited differences in the characters I put together. ie: "ok, this one is better at hitting things with a sword, this one has stronger magic, this one can open harder locks" but generally they all feel pretty similar in play style. I know that is a feature of the TES setting, but as evidenced by the encouragement they are giving to people using their highest skills more often this time around I also think that they mean for people to have a great array of choice, but to eventually settle on a particular playing style for each character and specialise for that build. So perks that let you add specialties in a very definite fashion would fit in well with what they are trying to do.
For example: for stealthy assassin types, perhaps there could be some sort of benefit/reward given for navigating through a dungeon or some other open quest without being detected, so as to compensate your more stealthy build for playing in-character. Perhaps maybe just bonus XP or bonus skill-use for killing an enemy without them detecting you? Sneaking up behind someone and shivving them/slitting their throat wouldn't normally give as much skill-up chances as a full-blown battle where you repeatedly use your bow, or magic, or sword & shield (etc), so it would be nice to get a perk that grants some sort of bonus. Kinda like the Mister Sandman perk does, but for killing unaware enemies, not just sleeping ones. Multiple levels of that perk to gradually add bonuses to damage, XP gain, stealthiness to encourage you to keep playing that character in that way. I could see a character I build that way playing very very differently to one with perks for toughness or an added mental reserve they can use to tap more mana at the expense of faster fatigue or a large hit/drain on fatigue (that potions do little to help) after a certain amount of time has passed.
And yeah I know it's adding classes into the game by proxy, but the difference is that it's more like gradual specialisation into the class/class combination you want your character to become, not arbitrarily picking it at the start, and in my view that is what the TES setting is all about.