My suggestion is an extension to the mining guild
Randomly generated structure you need to OSRS gold build. The faster you build the more xp you become. Jagex would have to work hard on their random generation of them. . .no 2 constructions should be the same!
Crafting: You are given a rnadom image you have you have to paint a specific image on a pice of armor. The nearer you manage to paint the armor such as in the example provided to you the greater the xp reward you get. A problem I see is the impossiblity of coming up with random pictures; some fansite can eventualy make a picture database. (this migh be deleted it seems a bit absurd and irrelevant to crafting)
My suggestion is an extension to the mining guild. The extension would be in an entirely new level of the guild, requiring 85 mining to get, but it wouldn't affect the original guild. This expansion could have 5 runite stones, 11 adamantine stones, 14 mithril stones, and 25 coal rocks. Additionally in the new addition would be a dwarf that sells dragon pickaxes, enchanted pickaxes, and mining rings.
Enchanted pickaxes cost 50k and get any ore in a couple of strikes but can just get 50 ores prior to breaking. When broken, bring it to the dwarf in the mines to cheap RS gold fix it for a fee of 10k.