I don't know how they would make these classes play different to the existing non-force using classes but you could have SIS for the Republic and Imperial Commando for the Empire and that would be a whole new storyline for each side...
This was exactly what I was thinking! I would also love to play as an Imperial Gun Runner (smuggler) and Republic Bounty Hunter. Implementing theses characters would not create any balancing issues since they would actually be identical to existing classes. It may decrease the ratio of force user players to non-force users adding to realism (I laughed when I typed that about scifi game). I'm also a big fan of adding more story options, but realistically I don't see them doing this in the near future.
I do think that a beast trainer and droid engineer are both possibilities for future class additions. You could be raising or building your companions rather than meeting them throughout the story. The big problem I could see would be that these classes would probably be heavily dependent on their companion for everything they do, which creates group PVP and PVE balancing issues. A creative solution may be for you to remotely control your companion during group content with a skill like (eye of the beast and optical slicing). CC's could function as they would on any other player.
side bar question: Does anyone know if disable droid works on companion droids such as T-7? I've never tried... but I think I'll give it a try tonight... that would be an open world PvP balancing issue.