Extending Amit's suggestion: Instead of an infinite grid, why not have them stacked?
Essentially, here's the idea: The (finite) grid is just one layer in an infinite stack of grids, sort of like an infinite parallelepiped in 3-space. At origin, there's a "down" portal and an "up" portal. Heading down leads to another grid that's twice the difficulty of the current one, and heading up leads to one that's twice easier; a few layers down and you'll be instakilled at origin-sub-5 before you can do anything. All the grids above origin are completely clear.
After a grid is cleared, a big boss battle that blocks the up/down portals occurs, and then the lower grid takes the place of the new one.
1) That might get a little complicated. What if the grid below is cleared before the grid above? Will the difficullty decrease as the grid moves up? What's the point of an 'up' portal if there's nothing to see there? Won't too many different up and down portals (not to mention combined with multiple servers) raise hell on rob's server banks?
2) It'll destroy the whole 'single battlefront' thing. The newblands will be the grids above, and the 'godlands' the grids below. Unlike if areas of higher/lower difficulty were in the same grid, changing between these 2 areas takes time (going through all the portals), and most people will just stay in their spot.
Well, I have a lot of invite codes and nobody to throw them at, so I guess pm me if you want one ^^
1) Server hell: since you'd get instakilled at origin around 5 levels down, there wouldn't be any need to maintain anything past, say, 10 levels down.
2) The idea would be that lower levels don't give better rewards, so there would be no incentive to fight there unless you were a scout trying to find some towns early on in game. Also, everyone would default to origin(0), so most people would prefer to fight on the easier levels above.