I love betas in a community tank. I have a 20g long with a male betta, a school of neon tetras, a small school(4) of Cory cats, a small pleco and some assassin snails.
For a 15g I'd recommend a small group of bottom feeders like Cory cats, loaches, otos or one of something larger like a bristle nose pleco and a small school of a mid level swimmer like tetras or danios
I also love the idea of doing a southeast Asian black water pool biotope based around a betta. That could be one betta, 6 harlequin rasboras and 3-5 ghost shrimp for example for a 15g. Here's a link to some informant about the biotope: