At the end of chapter 4, you're given one final chance to choose between Vash and V'Kai. Being locked into a choice at the end isn't a problem- but I and several others mistakenly thought there would be further opportunities to switch. I'm not too bothered by it, since I do have V'Kai over on my side, but it would've been nice to see it coming.

I propose that next year (if the Brotherhood returns, or if there's an event with a similar setup/style), the language of the final chapter be altered slightly to give the user a warning and let them know that this choice is final. Having re-read the text of chapter 4 on the wiki, I can't seem to find any indication in the chapter text that this is a final choice, meaning it's up to the user to guess whether or not they can continue switching like they have been.

So yeah, pretty simple suggestion! Pretty much a quick one-sentence addition to the last chapter.

*This will be your last chance to decide, so choose carefully!

^I think simply adding that would suffice.
If there's already a warning and I just don't remember/it's not on the wiki, feel free to ignore this!