Construction: A mystery along the lines of OSRS gold catapult cunstruction. Randomly created structure you need to construct. The faster you build the more xp you get. Jagex would really have to work hard in their random generation of these. . .no 2 structures should be the same!
Crafting: You are given a rnadom image you have you need to paint a certain image on a pice of armor. The closer you figure out how to paint the armor like in the case given to you the better that the xp reward you become. An issue I see is the impossiblity of coming up with random images; a few fansite can eventualy make a picture database. (this migh be deleted it sounds a bit absurd and unrelated to crafting)
My suggestion is an extension into the mining guild. The extension would be in a whole new level of this guild, requiring 85 mining to access, but it would not influence the initial guild. This expansion could have 5 runite rocks, 11 adamantine stones, 14 mithril rocks, and 25 coal rocks.
Enchanted pickaxes cost 50k and receive any ore in a couple of hits but may only get 50 ores before breaking. When broken, bring it to buy RS gold the dwarf in the mines to fix it for a charge of 10k.