Creating "Efficient Evocation" spell

[drdevoid]

I was looking for help developing a custom 9th level spell.

It's for a warmage that gets his last advanced learning at 19th level due to multiclassing.

I'd thought I'd base it on "Arcane Fusion, Greater" except 9th level and only for Evocations.

For reference, "A.F., G." allows a 7th and 4th level spell to be cast by one 8th level slot (but uses the normal saves)

Here's what I'm leaning towards:

1. Set number of spells (8th and 5th; Evoc only) use 9th level save
2. Set number of spells (8th and 6th; Evoc only) use normal save for spells
3. Allow a given number of spell levels worth of spells (12? 13? 14?; Evoc only) to be cast as desired, but limited to 3 spells or less.

Alternately, I could create an AnyEvocation type spell with an XP cost, but I'm not sure how to benchmark it. Use wish, Greater Anyspell, Shadow Evocation?

Thanks in advance,
Joe