That's an interesting question, I'd like to discuss it. I hope more people join.
As for tail shape, I'm fairly certain that it would be short (plakat-ish). Plakat bettas are significantly more active and energetic than their long finned counterparts and aren't as inbred (except for fancy variations such as "candy plakat"). Them being healthier, together with the fact that long finned variants usually have a hard time swimming and often pause to rest, makes you think that plakats would be far more successful at spawning, favoring shorter fins for the future generations.
As for colour... It's more complicated. Since you stated that predation is out of question, I think they would be bright as bright scales indicate health, therefore, are a preferred trait. They may be reddish as two studies showed us that female bettas prefer males with red colour. But they wouldn't be as red as the bettas you see in stores, whose appearances are acquired by artificial selection. Maybe a "cloudy" red, mixed with some other colours/patterns. But of course there would be bettas that look different (plus recessive genes are a thing), this is just what I estimate the majority may look like, considering the preferences of female bettas.
In case you're interested, here's an article about female bettas' preferences when choosing a mate.
I personally don't think they would evolve into different species as the environment and the conditions are the same, so there wouldn't be any pressure to change and adapt. That is considering they never run out of resources and living space.
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