Agency 13 is correct: The question is too broad for a specific answer.
What you put in any tank with a Betta depends primarily on your knowledge of of various species and their habits. And your attention to detail. An experienced fishkeeper can get by with a lot of "don'ts" that an inexperienced fishkeeper cannot. And by "experienced" I mean someone familiar with more than just Bettas.
Are potential tankmates territorial? Are they aggressive or peaceful? Are they fin nippers? Are they slow shoalers or so busy they'd stress out any Betta? What size will they eventually become? Big enough to eat the Betta and other tankmates?
What parameters do the various species require? Can you meet them? Do the fish need a strong current? Do they eat plants?
What do you want your tank to be? A peaceful tank of small-to-medium fish? Or do you like the displays and territorial behavior of Cichlids? Do you eventually want shrimp or other inverts?
It's impossible to suggest tankmates without knowing your vision of an aquarium and the parameters in which the fish must live.