Anything that complements the Grenadier's role of backline DPS and support. Therefore, definitely not Guardian, Ravager, or Duelist. Duelist MIGHT be okay if you're using it to cover your butt when dem Ravagers soar at you, but I don't see any use for it in PvE other than somewhat potent melee DPS.
That leaves ranged options. Bard is an okay subclass but from what I've heard it doesn't pan out in the long run, and with your HP healing crystal dealie you should have heals more than covered 'cept in dungeons.
Sorc is like everyone's favorite subclass cause utility is too good, with the CC, DoTs, and heal. Still, you already have a heal, I'm pretty sure you've got decent CC with the stun and ice.
Wizard is pretty good if you wanna add more nuking potential to your inventory. More damage/CC which works well with Grenadier.
And finally, Gunslingers are actually a pretty sexy cause traps are great when people step in em. They basically do what you do but with more mobility and utility and less damage. But your traps aren't as good as Gunslinger mains and have pretty high CDs, and DPS you have in spades.
tl;dr Depends what you want out of your Grenadier. Sorry, but it's true, lolol.