A good idea I actually saw on the PlayTheGame forums was to have a weapon or teleporter that works like the Kino teleporters did, but rather than take you to sections of the map that are unique, have them take you to sections of old maps.
I.E. You throw this black hole bomb and jump in it/teleport and you appear in the Power Room of Der Reise, or the Stage on Kino, or the Defcon Room in Five. Just iconic places from old maps.
And maybe you could make it that in each section of an old map there was the mystery box, and during the time you were there you could hit the box and get weapons specific to that map.
I.E. Der Reise- Monkey Bombs, Wunderwaffe, WaW weapons
Kino- Thundergun, BO weapons
Five- Winter's Howl
Ascension- Thundergun, Gersch Device