Yeah! I was thinking about it last night after posting, and I know it's not the best barometer for what we'll see in the game, but in the art provided, we're shown Frieza, Cell, and Buu, so I'm kinda starting to think my list would cut off at Kid Buu. ASW loves color palette swaps, so I think 17 will make it in as at least a swap for 18 at some point. Like you said, the roster is probably gonna be tight, and will probably focus on Z. I picked 18 since there aren't a lot of females to choose from and 16 for his more unique skill set, given ASW is probably the best dev when it comes to projectiles and special peripherals, they would probably make good use of those rocket arms... I think you're right and I honestly wouldn't mind if they kept it to Z and added on later be it through DLC or successors.
18 has a very feminine moveset, it's virtually impossible to have 17 as a swap, FYI.