How to make factions playable?

[gollum]

You need to check the startpos text file pikenier,
in there iirc, you can set any faction you wish to playable. There are three starting position files one for each era.

You can set the Aragonese to play a GA campaign as i recall, but you have to adjust the GA file, whose format is really messy. I would have thought it far easier to buy VI for your MTW, that makes Aragon playable and even provides special GA goals like the Sicilian VEspers. It has the VI campaign, fixes many things in the main MTW campaign and several bugs and also most mods are made for VI, so it gives you access to these.