The foundations of Onmyoji were very clear cut. Even though its playstyle was vastly different from I Am MT after generations of card games, the core was still identical, it was still getting cards, training them, and arranging a team.
The more difficult parts of the development process would be the quality of the artwork as well as making models. Onmyoji's models were exceedingly high quality, much higher than those in Warcraft and Wulin Legend.
There was a lot of work that needed to be done outside of the models such as background music, dubs, and character artworks. These were relatively to solve as long as he had money.
In Chen Mo's plan, Onmyoji was split into six different categories, Shikigami and Onmyoji system, quests, combat, socials (hunts, mail, chatting, etc.), Souls and shop related things, and other systems(summoning, codex, etc.).