The Seireitei is comprised of 13 floors, and each floor contains a captain along with their leiutenants. The reward for doing this team mission could be along the lines of...

+5000 Reiatsu/Health
+1000 ReiP
+1000 ReiD
+1000 Attack
+1000 Defense
Again, these are just suggested rewards, and there could be a time limit put on the mission (mabey once a day/week)

would need much bigger rewards for 13 boss fights if you mean the actual captains from the anime

XD boost edited along bigger lines >.> i forgot that's (counting squad 11's 3 leius)

bordering a little over 20 bosses >.>

I was also thinking of a Las Noches one...but umm....i cant figure how the bosses would work since i only know 4-10th espada lol

Very Interesting Proposition.