The Grand Inquisitor was my choice, as he's a good character who I'd like to know more about and because he also makes logical sense to make an appearance, though that can be said about any of these characters.

It'd be interesting to see what they decide to do, as I'm fairly certain that a sequel will arrive at some point. Who knows? Maybe all of these character will appear in a sequel, or if the rumors are true, at some point in the trilogy.

Maul, Grand inquisitor or ninth sister

Grand Inquisitor

Besides the characters listed of which of them I would like to see what about some EU/"Legends" characters.

Agreed. I'd like to see Kyle Katarn return at some point.