Other than targeted donations to giants to get an emblem to spend, or to speed up learning, there is currently no other worthwhile incentive to donate. Why not have the "Most Favored Giant" (MFG) reward a little extra something to those who choose to dabble in the skills that the MFG is affiliated with?
For exampe, if Pot is the MFG, Cooking and Cheffery related actions would yield slightly more *insert reward here* than a non-MFG action.
Obviously, the reward shouldn't be something that results in an unfair or lopsided advantage, but interesting enough to make donating to a particular giant interesting again. If donating had more incentive than it currently does, I'd be more inclined to donate than sell resources to vendors/auctions, which would decrease the amount of currants I would stockpile (which isn't hard at all at the moment).
Maybe the MFG status could be reset periodically (monthly?) to even the playing field, and a specific reward/perk could be announced ahead of time with a donation period. Whichever giant gets the most favor points after that period is over gets MFG status. Kinda like voting for an additional specific perk via donations.
Just throwing it out there to see if it sticks.