how do you think that would play out?

probably pretty stupid since the phoenix is surrounded in telepathic flames and symbiotes are weak to fire

If there is a force that will enhance the Phoenix Force in terms of costume probably the Witchblade, those tentacle like costume can make her sexy. I am leaning towards the Angelus symbiote but it corrupts but I do know that the Phoenix Force can over turn this glitch since PF is more stronger and more will power.

Instead of the Phoenix Force I go for the Quantum Force which makes the Captain Universe mix with the symbiote 

to powerful wouldn't work.

How come you think that? If the Quantum Force can inhabit any being why could it not inhabit a symbiote user, is unlikely because most user are evil or otherwise anti-heroes but if the situation calls for it, it needs to possess anyone. Sense the uni-force did possess a Badoom to prevent the original Guardians of the Galaxy from committing a genocide