For a game like that I'd say mix the elements of Six and seven for character custimization. Since 6 had one esper per person, keep it that way. However the amount of Materia depends on the weapon like in seven, anywhere from 0 - 8 slots.

And a list of Character ultimate weapons.
Cloud - Ultima Weapon
Barret - Last Score
Cid - Mop
Terra - Atma Weapon
Sabin - Tiger Fang & Dragon Claw
Gau/Gogo - Make one of them the character that randomly becomes another in a fight
Cecil - Ragnorock/Crystal Sword
Kain - White Lance
Edge - Murasame & Masamune
Squal - Lion Heart