Back to the original poster's questions...
Have you looked at what fish are easily available to you? What smaller fish catch your interest? I say smaller fish because many like to be in a group of 6 minimum but often a big school is more interesting too. If, say, you like cute catfish, corydoras are easy. It's better to get a bunch of the same species. For midwater swimmers I have always loved harlequin rasboras, pretty, very hardy & peaceful. Angelfish have different issues as do gouramis, bettas & guppies. There's the whole compatibility thing too, big fish often tend to eat or nip small or long finned fish. Temperature & tap water can be issues too.
Give us a hint of what fish you like most & we'll help you build a tank around them & tank size you'll need...or suggest a similar species if your space (or wallet) is limited. We all have our limits, I'm may never raise young discus again, keep fish that only eat live food or try to raise every single egg to adulthood, lol.
Don't forget to check garage sales for cheap larger tanks (or even free!). Ask friends, relatives & coworkers what they might have in their basement. It doesn't hurt to ask...I've given small-medium tanks away, nothing fancy, lol.
We want to help you! & we can!