It's a similar effort to creating a new monster for pen and paper RPGs. As you get into figuring out more upgrade options for them it gets closer to the challenge of creating a character class. But it's fairly manageable.
One of the guys that hopped on board right away created a Witch Hunter class. He used keywords and abilities that already existed for the most part, and that works well! Obviously, they aren't always cannon, but Relicblade is made to be a bit of a sandbox world. The wilderness is full of Infinity ruins, dungeons, and ice caves full of dragons and mummies and dragon-mummies. So I won't say no! Go nuts!
Relicblade is robust and flexible, and the core mechanic is based on difficulty and time. Actions that take a long time to do require more dice. The more difficult the action the higher number needed to succeed. And characters can focus by adding one extra die to an attempt (Adding more time to the attempt. See what I did there?:) ). So once you break that down you can gauge how skills should work like a DM would.