That requires infrastructure changes, which is pretty much impossible as nobody has a vested interest and its herding cats. P2P means over existing networks.
I forgot to write though: I am not convinced this is that big a problem in the real world, and there are other mitigations. One would be a "poor man's Tor," onion routing over just 1-2 hops. Since you are propagating and aggregating P2P anyway, its not going to be that expensive. Doesn't make DOS impossible but makes it tough enough to deter amateurs.