As an illusion pair,I can think of better.Add a good poison type which can switch in and take a fighting type blow.The other advantage of a poison type would be that it's walled by steels...Zoroark could hit them hard with flamethrowers from behind an illusion,as well as imitate their movepools rather well.Crobat would help with Lucario's ground weakness on top of a 4x fighting resist.Arbok could also work with intimidate cutting the opponent's attack.
Of course,Gengar has immunities to ground and fighting,which would link the three together rather nicely.
I'd say a poison type is the direction to look in first of all: things will become clearer once you've considered the other four members.No pokemon can be written off as bad if it's supported by the rest of the team.
Also,I'm free for flat single/triple wifi battles now(or normal rules PO/PS).
Anybody at all interested?