As others have stated, the main triumvirate (traya, Nihilus, Sion) is the core, and potentially Nihilus can be dropped against weaker bugs - the best options in most cases is to add either pre-taunting or auto-taunting tanks to spread damage around while they damage themselves from Traya’s Kit (this also helps the trio time to ramp up their own survivability from the kit interactions).
So Sith empire trooper is a great addition, Royal Guard and/or shoretrooper are both very good additions.
Another oft-overlooked addition is Sith Marauder, since he gains TM for enemy out of turn attacks (and the geos AI for the most part seems to favor attacking the weakest target available) every time a geo goes and calls a full team mass assist, Sith Marauder also gains full TM and gets to take a turn, doing fairly high single target damage to deplete their health pool (and recovering most of his own health in the process), makes for a surprisingly resilient ‘soft tank’ (since he seems to be favored by AI targeting as a generally low health pool character) that can also chip away fairly quickly at their overall health - Traya lead doesn’t rely nearly as heavily as other geo counters on AOE atttacks, since you isolate the Brute to stop the taunt and knock out Alpha first, and the team falls apart pretty quickly after that.
As a bonus, marauder is a fairly easy character to farm and gear up, and pulls double duty as a pilot.
(EDIT: spelling and grammar)