Adding in new Babies, In-Trainings, and Rookie Digimon are very simple. Assuming a quality sprite exists online, I can whip up a movepool, some base states, encounter rates, and evolutionary paths in about ten minutes.
Champions, Ultimates, and Megas, on the other hand, rarely have sprites that are going to fit into a 64x64 pixel box with only 16 colours. That means I'll have to spend the next few hours editing the sprite pixel by pixel until it fits the parameters. For example, you can see how much tweaking it would take to turn this:
into this: