I started down the 13th Age path with FG2 3.0. I extracted the 4E ruleset (since I figured it was pretty close and I could remove things that don't match).

It's working fine. I've updated the License, changed the logo.png, and even removed the wyvern background from 4E and put a 25% transparent 13th Age logo on instead.

I've also updated the main character sheet some. I have the Name, Race, Class, Level and the Ability scores rearranged to look similar to the 13th Age character sheet. I just need to link in the "basicnumber" field for the Level to the "abilityscorecheck" bonus.

But, it looks like others are also working on a 13th Age ruleset and starting from the Core. It makes me wonder if I should've started from the core or should instead focus on Numenera?

Numenera will be a good deal different and would definitely benefit from starting out with the Core ruleset.

Decisions, decisions....