Steven fusing with a full gem is something I'd really like to see in the show, so these are my thoughts on who it would be if it happened:
Garnet[]
After the events of Cry for Help, I doubt Garnet will want to do any more fusing for a while. When she was about to fuse with Amethyst, she said "let's just get this over with".
However, I think Garnet trusts Steven enough to know he wouldn't desecrate fusion like Pearl did.
Pearl[]
Fusing is an extremely intimate thing to Pearl, and I'm sure she couldn't bear forming Rainbow Quartz without Rose. We saw in Alone Together that she deemed Stevonnie "inappropriate", which might be from a leftover feeling that humans and gems aren't meant to fuse.
Amethyst[]
Out of the three, I think she's the most likely to be willing to fuse with Steven. She has a closer bond with Steven than with Pearl or Garnet, and their open personalities mesh well. She's also very willing to help him with his powers, like she did with shapeshifting.
Thoughts?