As we all know, Caesars Legion was formed in the conquest from 86 tribes, so how about 89? (Minus the Khans at this point)
There are about 3 tribes one lead by the burned man next by Daniel (I asume it's him as his name is in an achivement) and a red painted tribe which seems to be led by a man with a skeleton on his head and a unique power fist.
So what if we kill all of the leaders? Will we perhaps get the option to give them to Caesar? Of course Daniel and Joshuas tribe will not be open to the idea (Daniel seeming like a good guy as his tribe is not seen in combat and we all know Josh's story).
It could be a outside of DLC thing like telling Veronica about Elijah.

In any case, since this DLC involves Graham we got to get "some" new dialogue with Caesar and/or other legionnaires.

Better than nothing, it would be insanly stupid to meet the Legions number 1 most wanted man and not being able to tell them about it, I like how New Vegas DLC is actualy related to the the main game, other than Fallout 3's which just took you out of the game for a bit (excluding Broken steel).