For Ankylomon + Stegomon I really wanted to go for a dinosaur Digimon like Triceramon or Brachiomon but instead I’m going to go with Groundramon. It looks really tough and durable and retains much of Stegomon’s greenish coloration. Plus it has Ankylomon’s tail.
For Submarimon + Pegasmon, I’ve come up with a couple of choices. Without a level change, Seahomon (sea-horse), but honestly that’s more of a joke.
Indaramon is my pick though. It’s color scheme can come from Submarimon, it’s a horse like Pegasmon and as a stretch the shell shape is a bit of a counterpoint to the Digimentals’ shapes.