| Remco Sommeling |
Since the epic spell is most likely only gotten till lvl 21 you could boost power a bit more than the usual leap from 8th to 9th lvl spells which only take 2 levels of advancement usually.
I'd be more inclined to make a specific epic spell to summon rather than a summon monster X, summoning 1d4+1 creatures of the summon monster IX table seems a good choice. A bit more powerful than you would expect from a summon monster X, since it summons 1d4+1 instead of 1d3, but it is a spell for that creature type only which takes away versatility in exchange for power. 1d3 advanced or giant creatues from the same table is also a good choice, or a more unique creature like a half-celestial gold dragon.
| Inconvenience |
Last time I played in an epic game, my Fighter/Sorcerer/Dragon Disciple/Eldritch Knight developed an epic summoning spell. It summoned the spirit of the very dragon that mixed with his bloodline millennia prior. Statted the ghost dragon out then built the spell around its CR.
Basically I'm saying, if you're going to be making a personalized spell at ridiculously high level, I am of the opinion that it should summon a specific creature.