You could always get another tank, around 25 litres, and then get a betta. The danger is ending up like Littlefish with all her tanks
Most of these fish aren't in the profiles on here, so I'll give links to Seriously Fish.
A shoal of 10 to 12 of one of the following:
Soft water -
One of the
Boraras genus. I have seen in shops B. brigiattae (chili rasbora), B. urophthalmoides (sparrow rasbora), B. merah and B. maculatus. The sparrow rasbora is often mislabelled as chili rasbora because they are orange rather than red and people will buy them if they think they're getting a red fish.
Ember tetra are recommended for a slightly bigger tank but as yours has more height than is normal for a tank this length, you'd get away with it. Light orange in shops but deep orange red once they've settled in. [Also in profiles on here]
Sundadanios In the profiles on here as Axelrods rasboras in the rasbora section. Not common but I have seen them a few times.
Celestial pearl danio aka galaxy rasboras. [Also in profiles on here]
Scarlet badis, one male and a few females. Read up on SF before committing to these. They are bottom dwellers which need lots of tiny live or frozen food rather than flake.
A small group of
sparkling gouramis though as they are anabatids like bettas you'd need to leave an air gap. [Also in profiles on here]
Hard water -
Scarlet badis, as above
Celestial pearl danio as above
Endler's livebearer. males only or they'll overpopulate the tank in next to no time of you had females. [Also in profiles on here]
Then there are the rare livebearers such as
Heterandria formosa and
tiger teddies which both need hard water, though you'd need to source these on-line.
A small tank is do-able as long as you research before buying
fcmf and Luke have posted while I've been typing. Cpds are retiring fish so I would get a sizeable group of those rather than a small group plus something else. A group of 15 to 18 would be fine in your tank. Or a mixture of 10 cpds and 10 Boraras brigittae.