Asklemmy

I'll reply first on more general grounds. In my opinion, bots...

• should only reply to posts/comments when explicitly requested to, through a standard approach.
• should be properly tagged as bots, not just their username but also some interface element. And they should never behave in a way that mimics human beings.
• should have short, succinct output, that doesn't force other users to scroll past a lot of junk.
• should only have a descriptive output (it gives you info), not prescriptive (it doesn't tell you what to do).

Now, actually answering your question:

• a bot that links manga, anime and LN references to MyAnimeList, MangaUpdates etc. pages, like u/Roboragi does in Reddit.
• an unit conversion bot, like @[email protected] said, that also works for cooking units. (Specially when Americans say stuff like "half cup of onions", for me it's the same as "a random amount of onion"). I volunteer myself to help out gathering units for that.
• a simple Wikipedia link bot, that gives you a short excerpt of the Wikipedia link.