[junon]

Give people the option to create, import and export their own on the official site, and a means for browsing a catalog of them. You can infer how good they are (and thus their ranking) by how many interactions are made with the map or something, or just use a rating system that can be done anonymously.

No design by committee, and no random, unclear layers for the average user.

[usrusr]

The map on osm.org is leaflet based, pre-rendered png tiles. Each theme is a huge global tile pyramid, and because they are osm and not some random osm redistributor they better have those pyramids refreshed quite frequently. No, you don't simply create a "themehub" platform on the side.