Improved Spell Capacity (pg. 210 DMG for fast reference) to add a 10th, 11th, 12th, 13th, 14th, 15th level spell slot to your repetoire (can go higher, but you need Epic feats for some of these others). The higher you push the ceiling, the more flexibility you give yourself.
Various Metamagic feats and Improved Metamagic to bring down the costs - x3 to bring down most important metamagic feats' spell level cost down to +1 each.
Planar Turning is just worth having for making packs of great frothing demons wee themselves and vanish. Not a must have - but very, very cool.
You want Epic Spellcasting. And probably the Epic feat that grants +10 to one skill, so as to bump up your Knowledge:Religion to nice respectable levels, and perhaps your Concentration skill, for that in-combat casting you seem to intend to favor.
Remember, you are the glue that keeps the bits from flying off the other party members. The more Heal / Regeneration / Cure Critical Wounds spells you pack, the more vigorously your team can prod buttock. Empowered healing spells are just what the Cleric ordered. (So take Empowered Spell).
Make sure that either you invest in a ring of Regeneration to pass around the party for after-combat replenishment, or buy those wands of Cure Light Wounds in bulk. If you cannot count on party support, being able to Craft Wands and/or Wondrous Items is a pretty nice second string to your bow - enchanting your holy symbol to cast Holy Word at 40th caster level is just the niftiest thing. If this appeals to you, Craft Epic Wondrous Item may be in your future. (No great rush, could pick it up at 41 or 42.)
Don't be in too much of a hurry to enter combat - the smarter your foe, the more urgently they are looking to get to you and take you out of commission. Summon Monster IX is your friend, let your summoned pal(s) go chew on the bad folk while you keep your held action on hand to Silence pesky spellcasters. A quickened Dispel Magic is not a terrible thing to keep up your sleeve, either.
Combat - as long as you have access to Disrupting Weapon, Divine Favor, and Divine Power spells, a happy Cleric is you.
Enjoy your Clerical experience, best wishes.