It could be nice if it was possible to lend out a working force (assuming build slot is actually just a construction team with a number of tasks to do). If eg. a friend have been on holiday and have a massive upgrade to do (lots of fields, wells, mines etc.) it could be nice if it was possible to lend him your ressources for a limited time.
The adventure system could be used:
1) click a ressource icon.
2) select a number of ressources (eg. 3 geologs, 3 build slot 2 explorers) you want to lend out
3) Send them to a friend (like the general in adventures).
if friend accepts within same timelimit as pr an adventure, 1 gems/ressource will be deducted pr 5 hour the ressources are used, incl. the travel time from island to island (1 hour pr way). Max of 48 hours can be used and the one lending out the ressource naturally cannot use them while they are away (so if he lends out his build slots, he can't build anything untill they return). It will be possible for the one borrowing the ressources to return them before the 48 hours, but 1 gem pr ressource pr beginning of a 5 hours period will be paied (to BB). Any payment between friends can be done "privately" by normal trade (e.g. 10 gold/ressource for a fish).