What happens when a guy gets sent to a universe where multiple universe crossover with a object of his choice? He chooses the omnitrix and becomes a hero! Well, vigilante, but whatever. New transformations from all kinds of life, adventures unseen, and bonds yet to be made await!

Plot points: interactions with different species from different franchises leads to new transformations, from supernatural species to intergalactic.

Transformation causing affect to user, examples such as Big Chill being pregnant and Swampfire maturing causing user to getting uncontrollable acne, find ways to implement this into some transformations, such as Upgrade potentially developing a Symbiote like Ship.

Certain characters wanting something from the Omnitrix User, be it a fight(like Vali), a hunt(Kyber), the Omnitrix users affection in someway or form(Kuroka, Nekoshou transformation), or the omnitrix itself.

Don't be afraid to have the OC have a hard time at first, like major injuries to fatal, the omnitrix does have safety protocol to keep the user safe, even bring them back to life if they are killed.

In higher stakes situations the omnitrix tend to unlock transformations that'll get the user out safely, almost giving the perfect trasnformation to get out of a situation, so when someone prepares for the User to a high degree, your more likely to get a new transformation that'll handle the danger with ease.

Also, remember that the Transformations will be the peek of there biology, kinda like a Pokemon with max IVs and EVs, remember how Fourarms beating Looma with ease despite female Tetramands being traditionally stronger then males.

I'd recommend both anime and cartoons, be it the typical qnimes uses in the Union Academy fanfictions(please don't make it like those where the school dislikes nonpowered people), Pokemon, nothing op like one punch man or dragon ball, for cartoons, maybe the 2015 TMNT cartoon, Steven Universe, Generator Rex, Danny Phantom