The thing about her will is that all the characters interpret it differently and it's up to you to decide which one you agree with, which is interesting because you're always biased to which character you like the most. I always picked it up like this:

Zero thought she wanted a unified world, so he created the Patriots to achieve that, but wound up creating a totalitarian regime.

Big Boss thought she wanted a place where soldiers would always be welcome (which is why it upset him so much that she gave up being a soldier in the end). In the beginning of Peace Walker, the cabin seemed to encapsulate that idea. It seemed like a place where volunteers hung out and trained without being in any active conflict. This was continually bastardized, first with Mother Base, where he kidnapped people off the battlefield and forced them to fight on his side in exchange for living on his bitchin' base. Later, DD seemed to be a more violent base that was primarily focused on revenge rather than fighting for peace. And Outer Heaven/Zanzibar seemed like horrible military states where soldiers were forced to help BB take over the world.

Ocelot probably didn't care too much, he was more interested in carrying Big Boss's legacy, by trying to destroy the Patriots and create a perpetual war.

EVA is probably the only person to be told first hand what she wanted, but unless it's revealed in MGSV, then she probably didn't do anything about it. (Unless the will was to evenly distribute the Legacy, or turn Big Boss into the symbol of true valor, because she didn't want him to have the same fate as her)

Strangely enough, I think Solid Snake's worldview matches the Boss's the most. He fought to keep the world the way it was, to keep stealth nukes out of anyone's hands and free the world from the Patriots (Philanthropy's slogan was literally "To Leave the World Be"). I think she wanted the world to be full of borders and conflict, so that everyone could choose for themselves where their loyalty lies.

If we don't find out in MGSV, it's probably safe to assume that it's something Kojima wrenched into the plot as a personal message while still making a bit of sense. Maybe "The Boss's Will" is just code for something that keeps him motivated when he's feeling down.