This is a really stupid idea. I know it's a stupid idea. But it seems like it would be easy as hell to implement and would add an interesting touch to the game.
Modify each and every map to include a portal somewhere in the map that would take you to another map which would have portals to every other map in the game. Except the endings and other non-playables of course.
Another idea is that the access could be limited. Perhaps the portal would only open once you've completed all the missions there. You wouldn't be able to skip ahead - that would just cause problems. Maybe getting to the portal could require some great task, for example the door is INF/INF locked and a key must be found. Either one key per portal and it's hidden somewhere in the map, or one master key that you get at some point.
Why? Well, the obvious purpose would be to use old maps as storage somehow. Or maybe the map could be reset but all objectives taken out of it, so you can get stuff again and fight through it again.
I don't know. It's a totally random and completely stupid idea, but I can't shake it. Maybe some discussion from other DX-heads will refine it into something useful. Worst case scenario, we kick it around and make fun of it. I'm down for that too.