I actually do quite like the idea of there being serious consequences to using the tadpoles, and I agree that having it interact with the 'become a mindflayer' aspect of the ending would be cool. The thing is, if that's going to happen, I would say there needs to be another change made to the tadpole powers, which is to make most of them actually good. I went through my whole first playthrough without deliberately using the tadpoles to unlock more than 2 or so powers all game, not because I was determined to resist the influence of the mindflayers, but because most of the powers simply don't seem worth using. The ones that require an action are almost always worse than just attacking or casting a spell, in my experience, so the only ones I really used were the bonus on first attack or check one and the one free crit per long rest one. If there's going to be serious negative consequences for using tadpole powers, I'd say they need to be much more effective than they currently are.
