the easier class to implement might be a droid class with no mirror. Just a startarea with an imperial npc and a republic npc that you end up choosing on of them to follow. You might need to choose allignment in character select for guild reasons, but with a warning like "you cant join a guild untill choosing side at end of starterplanet" i dont see a problem.
Classquest would very similar for imp and rep, to shorten development time. Since starterplanets are so divided the first levels you would need to put a rep and a imp version of the same quest on some planets ofc. With similar classquests it would have to be more about gathering upgrades for yourself and getting items for your choosen faction from ex. pirates and smuglers than killing sith lords and jedi knights. Generic siths and jedis could work ofc, but the more story the sith´s and jedi´s get the harder they will be to factioncopy.
Droids class would be for droid"race" only ofc. In customisation you should be able to pick a few different types of droids. Been thinking about a change of chassis thing for when you pick advanced class, but i think it would feel wrong, at least for me, to change "droid" at level 10. Better to keep it in customisation.
I kinda like the idea of a class who lay down turrets and flamedroids and other types of minidroids. It could work well for a droid class.
Ofc the classquest beeing such a big thing in swtor lessens the likelyhood of us ever getting more classes. Maybe if you added a class with much less story. F.x. you could start the new class at level 40 and make a some requirements for starting that class. Then this new class would only get a chapter 3 and a prologue. During the prologue you could set some moral choices so you could start of with a fair amout of dark/light side points. The prologue would basicly be chapter one and 2 mashed together in one big voiceover.
I could see this as a Legacy Class unlock. Get an Imperial AND a Republic character both to 50 with legacy level (insert whatever here) and BAM!!! I believe any class they put in from here on out SHOULD be one in which faction could be chosen so as to limit the total amount of resources needed to add that class. Making it a legacy unlock could force some type of "sense" into starting at Chapter 3. Your idea of a droid class is excellent with the dialogue/moral choice limitations (as for as a "class" based story) and also helps add sense into getting the bugger at a starting higher level.
About the only purpose to my idea of it being a legacy unlock is to add SOMETHING that requires a higher level legacy since, as far as I know, above level 25, there is nothing. Also with making the requirements to have level 50s of both sides shows you have played through quite a bit so would be better able to choose which faction the new class would be out the gate by limiting the amount of possible remorse at playing choosing a faction you end up hating due to quest structure, planet order, dailies, or community.