So a problem I've run across is the difficulty in balancing citadel upgrades and purchasing items. Another facet of it is that Demigod kills lead to large gold bonuses, which lead to lots of gold, which leads to uber items and more kills. It's a snowball effect.
My suggestion is this. Instead of awarding Demigod kills with gold, award them with a new currency, lets call it Faith (Your followers seeing you vanquish another demigod strengthens their faith in you?) that is used exclusively for the Citadel. Gold would continued to be accrued from mines and the killing of creeps.
Faith could also be exchanged at a shop for gold, at a 3-1 ratio. Therefore, killing enemy Demigods would still provide an economic boost for items, but would be worth more in team upgrades. Gold could also be exchanged for faith at the same 3-1 ration (In essence, 3 faith for 1 gold, or 3 gold for 1 faith.)
I think this would effectively end the current situation where kills directly lead to increased superiority, therefore making the victor even harder to beat.
I would also suggest allwing players to purchase experience, in case one falls behind, and needs ro become competetive again. This could be done with either faith or gold.