Just resist the urge. Because you're not going to think to check for robots.txt, but you ought to, that's how we communicate we don't want to be involved. You're probably not going to bother to read the various terms of service for the instances you touch, many of which explicitly ask you not to do any scraping or automated activity beyond normal use of the service. You're not going to know to respect people using the NoBot hashtag that was meant to prevent automated follows. You're certainly not going to parse user profiles and pinned posts to learn how people like to be approached, what will get you blocked, or even think to heavily throttle your activity because instances are falling over in response to load.
Whatever your thing is, make it 100% opt-in. Make it appropriate for a significantly more at-risk user than you are. Make sure it forgets things, purges info about servers it can't contact, can't operate in any sort of logged-in mode where consent is an issue.
We will straight up help advertise your cool thing if it respects users properly and takes the time to consider the safety and preferences of every person involved. There are a lot of fun, thoughtfully-designed toys! And there are a lot of people really tired of having to come and tell you off when you wanted to help, honestly. Help yourself and ask around before you flip on your cool new thing, let folks point out what you're missing.
