A is for Altair, a major star system that may or may not have been in the Federation during the time of TOS
B is for Bolius. The musicians there really know how to sing the blues.
C is for Cardassians. They have spoons on their foreheads.
D is for Deneb IV. James Kirk and Gary Mitchell visited it more than a century before Jean-Luc Picard did.
E is for Ekos and it's Nazi copycats.