Simply make God playable as a character
He would have uber powerful spells like Oaks pentience except double damage, armour reduction, vampire effect and attack speed slowing as well.
He would have 5000 starting health, 7000 mana (even though his spells only cost 50 mana tops) and 2500 armour
He can summon ArchAngels which are effectively Regulus' but with swords and double the stats
He also gets a 90% gold reduction on all items since he made matter and all
And of course, only has one level 15 ability - Implode
Implode - Once enemy Demigods come within range of God they implode out of his sheer awesomeness (50 Yard range)
Edit: Many people complained that God was in need of a buff so has been given the level 1 ability Heavenly Judgement. This calls down a metoer from outer space to crash down upon the enemy dealing 1500 damage to anything 10 yards within the target range. This damage increases 1000 AUTOMATICALLY every level.
God only needs 10 exp to go up a level
He needs a weakness. I think we can exploit his greatest weakness of all: Bad planning. Despite knowing absolutely everything and being able to do anything, he somehow didn't know of even deduce the outcome of the whole tree-in-the-garden thing, or the whole sodom and gamor thing, or even the having-to-flood-the-world-coz-everyone-decided-to-be-wicked thing.
This should be codified in-game somehow. His weakness will be that he will never respond the way you expect him to when you click or use an ability. If you click on a near-dead demigod, God will turn his attention to the farthest, weakes creep. If you cast diving might, and have 100% health, God will use a health potion. If you retreat, God will grow a bush, then set it on fire. When he ragequits, the whole playing field will get covered in a flood.