As others mentioned, the Annihilator could be a good choice for a new assault. Also, the Nightstar is a possibility too, however that thing is a monster in the firepower department, so I don't know if it would be too much power-creep. I'd like to see the Nightstar in the game (it looks amazing), but at the same time, I'd be a bit nervous if it did.
Also, the Nightstar would require Lostech to be loadout correct (something that makes it even more spooky). The equipment it would need is in game though, so it wouldn't need anything above and beyond (like LBXs or Ultras).
the nightstar 9J is a beast to have on your side or see with the opfor.
2 gauss and a erppc is a formidable loadout with lotsa armor for preotection.
most of the equipment for the nightstar is in game but it needs an XL engine which is not.
i doubt we will ever see it until/unless HBS makes XL engine possible in vanilla.