I agree Watcher could be interesting and relevant to the story, and would be interestingly hard to stick to with some genuine conflicts between its tenets and the available major story choices. In addition to what’s mentioned here, Wyll could possibly be ruled out as a companion as someone who trucks with a fiend, unless the fact he regrets his choice means that there is a path to working with him. There would at the very least need to be some negotiation and debate.
Would accepting help from Omeluum be ruled out though? He’s not a fiend and, unlike Ethel, there’s no indication I can see of him having an evil agenda. I know Watchers are meant to protect from extraplanar threats, but does that mean they need to treat every representative of an extraplanar race as a threat and refuse to deal with them?
My other pick was Vengeance, and I hope we’ll get to choose an interestingly relevant enemy (this one would make more sense to pick later, at level 3, so you can know what you’re up against).
I can see any of them being interesting, though, as long as they’re implemented well. Conquest is probably the one I’m least fussed over, and I’d rather play an “evil” paladin as vengeance, glory or oathbreaker, but I definitely understand why folk want it.