Riffing here, as I think the idea of buying (or selling) a one-off add-on is problematic (how on earth do you control usage/quality?), but how about a subscription-based MODx package provider (with a user/key combo), with devs getting either a per user, or per download fee. It could work a little app-store-like - MODx core team control quality and admin, take a cut for doing so, devs get exposure and simpler management.
For all I know this could be what’s planned anyway, but it seems to be easily ’developable’ - pun intended.
To respond to someone else, I know that tip-buckets/bounties have always been used, but I’m thinking more of a concerted programme of crowdfunding. I envisage a system where devs auction work (ie - I’ll build X when bids match $200), and I can bid (add ’votes’) at, say $1 a go, as an individual. It wouldn’t take a lot of sponsors to get things moving on several fronts (and persuade other devs to learn the core). Or, we could do it en masse as a community - like the awesome
http://www.quirky.com - post idea, crowdsource specification, devs pitch costs, crowd raise money. Essentially, it’s just formalising what goes on here already.