Thanks for the detailed feedback,
@NotAYakk!
I'd drop this and give them scent? It is more fun.
I considered just Keen Scent in the original version, because it was indeed more fun and flavorful. But it doesn't appear that any PC race you'd expect to have specific keen senses (like sight or smell) actually has them; instead, they have access to full-fledged Perception proficiency. So I bowed to the standard.
This seems sub-par at this point. Burning an action for a HD is pretty meh in combat. Out of combat it just means that your short rest for HP recovery is shorter than 1 hour.
And, what little use it has in combat doesn't scale; 1 action for 1 HD+con at level 1 isn't bad, but by level 5 it is a very stupid move.
Note that this is basically how Dwarven Fortitude (from Xanathar's) works. Both use your action to heal, but Dwarven Fortitude packages it with a Dodge action. And since that's expected to be acquired at 4th level (dwarves don't get a starting feat under 2014 rules), downgrading the base version to "you still take an action to heal, but without the Dodge benefits" seemed logical. (Trollish Fortitude, of course, is basically Dwarven Fortitude with some troll bits tacked on.)
As an idea, what it was:
* As an action you can start to regenerate. Roll a HD adding your constitution bonus to each die and add that value to your regeneration pool. Whenever you have points in this regeneration pool, lose your proficiency bonus points from the pool at the start of your turn, and heal up to that many HP. When you take fire or acid damage, you do not heal from your regeneration pool until the end of your next turn, but still lose points from it.
This is both weaker and stronger than your above version. It is weaker, in that the HP comes gradually, and doesn't even start healing until the start of your next turn; it is stronger, in that you burn 1 action and get the benefit for a while. It also acts more like trollish regeneration.
I like the idea of a healing pool, and appreciate you working out the mechanics, but this honestly seems kind of complicated for a 5E trait. Especially, again, when Dwarven Fortitude is clearly more the officially intended approach. (That, or the goliath's Stone's Endurance.)
I actually did look at the fighter's Second Wind and the paladin's Lay on Hands feature as possible models for a "regeneration pool" at one point when working on version 2, but it seemed to step on those classes' turf too much.
A problem I have with the half-feat model is that you could easily max out your strength or constitution before you get such a feat. I'd rather these feats be bigger and not be half feats?
1) Assuming your troll PC starts with Con 17 and Str 15, you'd need three of these feats to max out your Con, and picking Str for the other two would only get you to Str 17. You could pick ASIs instead of some of these feats to get both to 20, but it'd be at the cost of the full troll features. (A troll fighter could also burn up some of their extra feat slots to max their Str or Con a little earlier, of course.)
2) Don't blame me for the half-feat model, that's the standard for most feats in 5E. And if I piled too many of the non-ASI features together, the feats would be too powerful for their level (unless I delayed these super-feats to even higher levels than they are now).
Bite is a ribbon. Make it a bonus action to make it matter.
I'm inclined to keep Bite as more or less a ribbon, but if it seems balanced with other Level 4 character options, maybe.
I might add bugbear style reach. Long arms.
The bugbear's weird long arms, I'm pretty sure, were an attempt to give them something Large-ish as a replacement for the monster bugbear's Brute trait. (In the process, giving them a trait that was distinctly un-bugbear-ish - until MOTM, I suppose.) Since the troll is designed to get actual Large size, not inclined towards such a weird workaround. (It fits them even less than the bugbear, anyway.)
Large is the only thing here worth a feat. And as it depends heavily on that sidebar, you should detail it.
I'd rather keep the sidebar, so it can be applied to any other Large PC races that might occur later on. (I'm also skeptical that it would be listed out in the race writeup were this official.)
Eating up all of the feats a PC gets (more than even) isn't great in my opinion. Especially as you are doing it with half-feats.
No, it's not at all ideal - I originally intended to only do 2-3 feats (since a CR 5 creature like the troll should only be about an 8th level character, by my calculations). The problem is balancing troll regeneration with the two means of PC regeneration that exist in the core rules, the very rare Ring of Regeneration and the 7th-level Regeneration spell - both of which should only be available to PCs of level 13 or higher.
Again, I could condense the features and make them available as only higher-level feats, but that means lower-level troll PCs are stuck with the fairly limited base features.
Half feat, 1 point of AC, and body parts. A bit slipshod, honestly.
By T4, I'd make the regeneration occur whenever you are at half HP or less for free.
I'm actually considering moving the reaction regeneration to 12th level instead of the +1 AC, and making the 16th-level feat permit per-turn regen (like the actual troll).
I'll probably do a version 3, but I'll wait here a bit longer for any more feedback.