This is the ONE THING that gives me pause about predicting Apollo will be our 5th soul retainer...his bow. Unless SSM are keeping that weapon under wraps, I doubt we'll be seeing it and therefore it makes me wonder if Apollo would FIT as the 5th God.
Apollo is all over the GoW fiction, though I can't recall if he ever shows up outside of statue form or 3rd party reference. But he makes the most sense for several reasons that i don't care to repeat again.
HOWEVER...Supposedly Zeus' "main weapon" is going to be the oversized boxing gloves...so a 5th God with it's own "main weapon" makes a ton of sense. Ares - Hammer, Poseidon - Spear, Hades - Sword, Zeus - Fists, 5th God - ???
I still say it'll be Apollo. He's the only major God without his own Armor/Weapon named after him. Although it COULD be Kratos...