Mangoose
Arcane
Oops, didn't see this, thanks!Powertech if you going to have a Gunslinger.
I've leveled one of each type:
Powertech (Tank/DPS/DPS) - close quarters, mainly 4-10m ranged. Tank spec is probably the easiest tank spec to play as you don't have to worry to much about defensive cooldown abilities, all shield & armour. Very easy rotation, but also very boring. Pyrotech DPS spec is good and strong, DoT focused but good at AoE as well. Rotation is not too hard to master. Advanced Prototype is powerful as well but has a hard rotation to master to get the best from it. It is one of the strongest DPS specs though.
Mercenary (Healer/DPS/DPS) - all ranged, 30m. Healer spec is interesting as you have heavy armour and good mobility with it. Haven't played one much though so I don't know how well they play. Arsenal spec is the easiest DPS spec to play and also very boring. The rotation is very simple and hard to fuck up. Innovative Ordnance spec is very much like Advanced Prototype but ranged instead of close quarters. It's very strong, probably the strongest DPS in the game, but it's also the hardest to master as the rotation is a little odd. It also doesn't AoE that well.
To be honest I found Powertech more fun to play as the rotations felt better to me. In PvP Powertech is king without doubt, tank spec is almost unfair with how many ways you have to annoy the opposition, and the DPS specs, though not very bursty, are dangerous and you are very tough with it. I always found Mercenary to be a little weak in PvP though. It plays a lot like a Sorcerer; if you get left alone you can murder people very quickly, but if not you're going to have a hard time doing much.
I've been leaning towards Powertech as I've been reading it has an interesting "mid"-ranged playstyle. And the AP animations I saw were pretty cool.
On the other hand I just saw a video with the new KotFE abilities, and the Mercenary launchback looks pretty badass and I don't mean just for SWTOR. (The rest of the animations are both clones and look blah, except for some reason the Commando and Mercenary ones).