You wanting to step out of the Betta box?
Well...I would suggest my first fish...and my longstanding favorite.
Danios. (The little ones not the giant ones!) If you find a store that has a bunch look for some long finned ones...some blue ones...pearl ones...leopard...zebra...they're all great
"Fish on crack" totally different than your bettas...but they're VERY entertaining to watch and definitely have a huge personality.
If you have a heater etc cool
They like heated water, but can live in cool water too. I would say you could easily put about 6 in a 3 gallon with a little bottom feeder or some shrimp. I kept 5 in a 2 gallon and never had any problems. (I also didn't have a bottom feeder...they kept the gravel pretty clean) Goldfish are very messy for their size...danios are the opposite, they're very clean for their size...which is why I suggest that you can put them in a smaller tank. Maybe only get 4 (keepin in mind that they get to be about 2 inches)
Anywho...think about them
I love em.
Edit: wow everyone posted at the same time! Danios will eat pretty much anything
flake food is fine, probably not the betta pellets, but they might enjoy those too lol
Different fish "school" with different number of fish...I think the fewest I would ever keep together of a schooling fish would be 3