I'd make it able to create it's own alien with a selection and/or AI. For the AI part, if the Omnitrix saw, for example a fire, it would instantly transform Ben into a alien with hydro powers, and also slow down time for Ben so that he could formulate a plan (he speed is still slowed down, just his cognitive functions are increased). For the selection part, the Omnitrix can combine DNA to form a mutant. For example if there is a large scale threat on the other side of the universe, Ben can combine Toku'Star DNA, Sonorosian DNA, and Aerophibian DNA, to form a mutant that retains the full powers of each alien.