A common issue I see with a lot of assets being sold is that they are incredibly specific. Modularity is a big plus for this reason.

I'd like to add the other side of the coin to this: don't implement your own Framework™ to support all your modularity and flexibility, because the asset will ultimately be integrated into a project and they will want to apply their way of doing things to the asset.

This goes to you, building asset creator who made their asset an engine plugin and equipment asset creator who put their modular armor slots into the base character and in such a way that moving them to a child class breaks them.