Summoning other creatures into battle, based on spell power and/or hero level.

For example, Elves or Grand Elves, Unicorns, Phoenix, Nomads, Rogues etc.

I personally think Elementals is enough. Even Ghosts sound like a cheat, although this can be enough.

What's the actual suggestion? To have 62 new summon spells?

It's certainly quite possible to summon different kinds of creatures, as the Pandora's Box shows.

No. Anyway I agree with Orzie that summoning creatures other than elementals doesn't suit the game.

Isn't Pandora's Box a H3 artifact?

It is, but Darmani made it another way: it summons random stacks of lvl1 creatures at start of the battle for each battle. Creatures disappear just after.

For me, it is a nice solution, yet I want to unsee the graphics for it. Perhaps I'll get back to it because I will anyway be working for Pandora's Box for H3SW and can make it in classic H2 resolution too so PI will also benefit from it.