Sorry if this is a dumb question, but what is actually possible and where are the limits?
Pretty much anything
What sort of limits do you mean? Rate limits? Or what can/can’t an extension do.
At the end of the day a Twitch Extension is just a static website. That can load data from an API.
Some content restrictions are covered here Extensions | Twitch Developers
Guidelines and policies Extensions Guidelines & Policies | Twitch Developers
Technical reading Required Technical Background | Twitch Developers
I’m learning Twitch extensions so I can’t help with the technical details. But if you are wondering what is possible, I would recommend looking at the showcase which metions GIPHY](Bring GIF magic to any Twitch channel with the GIPHY Extension | Twitch Blog) and Pokémon Badge Collector
ALSO, in the Twitchdev discord there is a #developer-showcase room which is very motivational seeing the cool extensions being created.