If you enjoy the flute you could go for one of the 'endangered' wind instruments i.e. bassoon or oboe. I personally prefer the bassoon, because it is one of three instruments that can blend with pretty much anything (horn, which is my instrument, and cello being the others). There also aren't many bassoons in an orchestra but ANY amateur orchestra will be more than happy to take any bassoonist (and oboist) they can find because it is so rare for people to take it up these days 🙂 I would say this is the case for horn, percussion, bass and viola, so any of these would be good options as well.
It's also ok to spend some time (whether that be hours or months) playing around with different instruments to work out which ones you really vibe with - I've played many instruments over the years but the horn was the one that I completely fell in love with and has stuck for nearly ten years now