I think that it's a bit too predictable that the M60 was meant for the DEA. Obviously, wherever he's been, it's been nearly a year (indicated by the birthday breakfasts) and judging by his tattered look, he's been on the run or at least in hiding. I can't think of a single thing he would get out of comitting a one-man assault against a police force... it's Walter White, not a coked out Tony Montana.
If anything, I think it'll be the [previously] rival cartel from Season 5 that Walt convinced to do business with him; they've probably got some grievances with the fact that Walt has stopped manufacturing for them. Hell, Walt may even fall into a situation in which he'll be helping Hank so far-- and the DEA by extent.
After Hank became so disenchanted with the rules of law enforcement, closing his Heisenberg case, I wouldn't be surprised if the main goal became not to get rid of Heisenberg, but the main manufacturer (I'm not saying Lydia would slip and given away the formula... but Lydia would probably slip and give away the formula) of the blue meth in general-- Walt may be the only person that could make that happen.
