In my opinion, the Saboteur has one of the worst power synergies in the entire game. It's really hard to name one that is worse. None of his powers synergize with each other at all. At least classes like the Marksman, BF3 Soldier, Vorcha Soldier, etc., can avoid problems with power synergy entirely by simply skipping a power. The only synergy that the Saboteur has is Sabotage with his Heavy Melee.
Anyway.... my answer to your question is probably the Salarian Infiltrator or the Geth Engineer. The SI because I keep going back and forth between single-shot sniper builds, multi-shot sniper builds, shotgun builds, and a Crusader build, which are all different. The GE because I can't decide if I like healing turret, flame turret, or just a few pity points in the turret. I change my mind frequently on that.
You know nothing about the Turian Saboteur if you think it has no power synergy.
Step 1: skip Turret. It's stupid and useless (yet it's still better than some pugs)
Homing Grenades and Sabotage synergize quite well, actually. Sabotage takes 1.5 seconds to prime for a tech burst after casting it, which works with the travel time and casting animation of Homing Grenade. That will get you a 6+6 Tech burst and some heavy damage to both shields in AOE, and armor against bosses.
Sabotage should be spec'd for Duration, Explosive, and Tech Vulnerability. Duration is useful for hacking boss units to keep them off your back while you mop up some mooks. Tech Vulnerability may or may not synergize with the team, but it certainly buffs your melee damage considerably.
Homing Grenade should be spec'd for damage, burning damage, and armor damage and mitigation. Armor Mitigation allows for flexibility in the types of ammunition you choose to carry with you, depending on the weapon you are using. Also, homing grenades spec'd this way are actually very powerful for taking boss units down. The armor mitigation makes weapons like the Hurricane very effective sidearms for taking down boss units without even needing AP Ammo IV.
Passives are full on weapon damage and headshots. This character is not so much an engineer, but more of a soldier. Sabotage is used to stun mooks for super easy headshots, making weapons like the M-11 Suppressor and the Graal Spike Thrower very fun weapons to use on this character.
Fitness is optional. I suggest a melee build because it gives you another avenue of dealing damage to make this character even more versatile than it already is. Additionally, it's synergy applies to weapon damage, which lends to it's already present strategy as a soldier.
I cannot emphasize enough that the Turian Saboteur is as much a soldier as all of the other Turians appear to be.