So! The exact mechanics of this would probably be something to discuss once you're in game and I can sit down with you to bash out the details, but based on what you've said here;

A generic 'monster summoning' power that allows her to summon more/stronger monsters as it increases in Rank would be fine.

Fusion would be its own separate power, with its scope limited by its rank - the exact details of how would depend on how the monster summon power works and we can chat about it once you app!

I fully admit the mod team knows zip and zilch about YGO past like, the Battle City arc of the very first anime so we would need a bit more info from you about how it works in canon before we nail it down but hopefully this is a helpful start!