I'm sure Ryza's defining "gimmick" would be synthesis/alchemy (obviously). If I had to come up with a concept for her off the top of my head, it'd be something like this:
Ryza's down special spawns an alchemy pot and has her thinking about what she should make. From here, the player can press A, B, X, or Y, which all add a different ingredient. The unique ingredients mapped to those four buttons are always the same, so there's no randomness to the move. During this move, the player can only pick two ingredients to throw into the pot, but each combination produces a different item. Just to make things simple, I'll say she's able to make actual items from Ultimate itself, even if they're turned off in the match. Let's call this move "Synthesis".
Alright, so as an example, you use synthesis (Ryza's down special) and press "A" followed by "B", which causes her to toss the "A" and "B" ingredients into the pot. This results in Ryza creating a Fire Flower, so she's now able to pick it up and use it in battle. "A" and "B" would always produce a Fire Flower, so once the player learns that recipe, they can now create a Fire Flower anytime they want.
This would make it so that players can memorize recipes for Ryza's down special, and use that knowledge to their advantage.
"A" + "B" = Fire Flower.
"A" + "A" = Fire Bar
"B" + "Y" = Ray Gun
"X" + "A" = Bob-omb
"Y" + "X" = Home-Run Bat
And so on and so forth.
The animation for the move would have to be pretty quick to have it work, and you should be able to cancel the move by shielding.
Other than that, she fights with a staff and has some special attacks and whatnot, so those could be used to fill out the rest of her moveset.
That's the best I got, lol.