If you pick a base character then the prime gets removed as well, the better version cant exist without the base version, however if you remove a prime version then the base character can still exist. My personal pick would be Valkyr due to how [IMO] useless she seems
I really think Limbo was a bad idea in general. He’s a very clunky frame that causes frustration for the rest of the squad, with the amount depending on how much the Limbo player is willing to actively try to mitigate it. When his abilities are able to consistently function they have also historically completely trivialised game modes. There’s not even a good way to fix these issues without turning him into a different frame entirely.