Extra reactions seem perfect honestly. The indomitable feature is a bit lacklustre, personally I would have made it so it could be used 3 times from the start or make it so it can be used after a short rest instead of long rest.
Extra reactions, especially with a caveat like "You can only make 1 opportunity attack per turn", would also make the Protection fighting style less inferior. If you can only make 1 opportunity attack, but you have extra reactions, and an enemy makes multiple attack on your ally, you can just decide to attack once thanks to Sentinel, then use Protection for another.
....a limit of 1/enemy would help head off bag-o-rats style silliness....Nyeh I would allow opportunity attacks if you have a reaction available its not like being able to make more than 1 is some horror story.
Sure....a limit of 1/enemy would help head off bag-o-rats style silliness....
As opposed to a minor speed bump... yeah...but, yeah, apart from mechanical artifacts, part of the point is probably to take multiple OAs, or the like, so the threat the fighter poses remains credible.
....a limit of 1/enemy would help head off bag-o-rats style silliness....
I figured that was the intent.that's what 'once per turn' mean, not 'once per round'.
Tiger Woods and Michael Jordan normally play against humans.Reskin the Diviner Subclass ability Portent and have it work mechanically in exactly the same way (because that is effectively what happens in our world).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.