Before we go off on a tangent about how a surviving Pat Tillman would reshape the NFL... Would he even be able to make a comeback. I mean even if he stayed in shape and somehow managed to practice his routines for the two years he was in service, he would still be playing 2001 style football in 2004, he could as well have broken his back on his last game and then spent two years in and out of a hospital getting back in shape.
Also, Tillman was no Manning brother. It is not as if two years in Afghanistan would give him superpowers. He'd still be a fairly decent player but by now two years older then anyone comparable, and in the NFL, unless you're Tom Brady, two years really is a lot of wear and tear and seriously reduces your average playing time.
2001 football in 2005 actually. He enlisted in May of 2002, so assuming he doesn't reenlist or otherwise end up with some sort of extension, the soonest he could possibly return to the NFL would be the 2005 season. If he doesn't actually leave the service until late in 2005, he might even have to wait until the 2006 season. Considering many NFL players don't last more then a decade or so unless they're very successful, or unless they're really lucky about being kept on as bench warmers or one or two odd positions, a 3 or 4 season gap could well kill any future NFL career