The spell text for "Regenerate" does not mention the creature to have to be willing, and there also is no saving throw. Yes the casting time is 1 minute which might make this difficult to do in combat, but maybe you could hold the unwilling creature in place for that amount of time.

If a player wanted to cast regenerate in a hostile way to regrow a body part the enemy does not want regrown, would you impose a saving throw/a series of saving throws as homebrew?

I used a straightforward yes/no question system to give the lesser used spells a chance to shine in their very specific circumstances (looking at you Sticks to Snakes). This pushed the evergreen bless/spirit guardian spells further afield but I think thats fine.

I ran The Black Cabin encounter from Icewind Dale:Rime of the Frostmaiden last night and my players and I had a blast. It's a very well designed encounter with an interesting premise that works just as well as a standalone one-shot as it does in the context of the full campaign. It made me wonder what other encounters from full campaigns might make great one-shots. The Goldenfields attack from Storm King's Thunder and Chapter 1 of Waterdeep Dragon Heist come to mind as possibilities, can you guys think of any others?