I tend to do themed runs (monotype, monogen) on occasion, but I settle on a team after 2-3 playthroughs. The non-themed teams (like the one I'm using atm) are based on a Fire-Water-Grass and/or a Dragon-Fairy-Steel core for stability. I try and avoid using NPCs' ace 'mons as much as possible, but if I like the Pokemon enough then I'll use it regardless. A Reborn / Rejuvenation-specific rule I have for non-themed runs is to only use event Pokemon.
As of the end of E18, my team is:
Salamence
Clefable
Empoleon (completes the Dragon-Fairy-Steel core)
Arcanine
Rotom-Wash
Naganadel* (it'll be my ace since no other NPC uses it, and I can't let it waste after all that effort)
*The last slot was previously filled by the mystery egg reward (Volcarona) and my first mega (Heracross).