An ability to ignore XP cost of non-epic spells, to make sure he can use Miracle and Wish without backlash.

The problem is that if we optimize him too much, he will not only be a threat, but also an unbeatable boss.

I didn't complete the build, but these feats remain interesting : Improved spell capacity, Improved metamagic, and Spontaneous domain access (luck). The first two will allow him to cast quickened 9th level spells more effectively, the third will allow him to spontaneously convert prepared divine spells in spells of the luck domain, thus allowing him to convert Implosion into Miracle if he needs to change of strategy.

Some arbitrary epic spell, according to his spellcraft modifier, may also be appropriate, otherwise Antimagic Field is so easily cast.