I think it'd be awesome if it was some sort of combination between the Gestahlt meter from FFXIII and Devas from Guild Wars.

Basically you always have access to the summons you've unlocked, but you choose to channel the summon's power (like Devas) to gain a new set of skills/abilities (for example, channeling Shiva would give you a bunch of ice spells) for a little bit (like 10-30 seconds depending on the power). To perform a "true" summon, there's a Summon meter while you're channeling a Summon that fills up as you do specific things (like damage enemies, crit, crit magically, heal allies, take damage, etc.) and when the gauge is full, you have the ability to actually summon the ... eikon and it'll use its "ultimate" ability (Diamond Dust, Hellfire, Tsunami/Tidal Wave, Megaflare...)