introduce some kind of a prize to stimulate people 
like: post for the next 20 years every 2 weeks and get a free Amazon card!
also topics like this are usually fun to read after several years have passed,just look at the pizza for bitcoins topic
I consider your idea rather smart or stupid. Yes you can stimulate their motivation for the prize but thinking about you will gwt it for what? 20 years?! Who can wait that long for an amazon card! You can earn here by signature campaign and buy yourself that card in less than 20 years. Plus, who runs a contest for 20years? Maybe the dares you and your buddies take. like: post for the next 20 years every 2 weeks and get a free Amazon card!
also topics like this are usually fun to read after several years have passed,just look at the pizza for bitcoins topic