Construction: A mystery across the lines of OSRS gold catapult cunstruction. Randomly generated structure you have to build. Jagex would have to work hard in their arbitrary creation of them. . .no 2 constructions should be the same!
Crafting: You are given a rnadom picture you have you have to paint a certain image on a pice of armor. The nearer you figure out how to paint the armor like in the example provided to you the better that the xp reward you become. An issue I see is that the impossiblity of coming up with random images; a few fansite may eventualy make an image database. (that migh be deleted it sounds a bit absurd and irrelevant to crafting)
My proposal is an extension into the mining guild. The expansion will 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 rocks, 11 adamantine rocks, 14 mithril stones, and 25 lava stones. Additionally from the new addition are a dwarf that sells dragon pickaxes, enchanted pickaxes, and mining rings.
Enchanted pickaxes price 50k and receive any ore in a couple of hits but may just get 50 ores prior to breaking. When broken, bring it into the dwarf in the mines to cheap RuneScape gold fix it for a charge of 10k.