I could see me running out of regions, especially as some of them would be being used for other things besides common events.
There's an easy solution to this if all of your maps are the same size. You can create only one common event to handle all map transfers.
(1) Determine which direction to travel by looking at the direction your character is facing
(2) Use the direction and current map ID to determine the Destination Map (suggested method below)
(3) Setup a variable for Destination X and Destination Y
(4) Keep your current X if you're going N/S, or your current Y if you're going E/W
(5) Assign the other variable depending on which direction you are traveling
(6) Transfer using the Dest M, Dest X, and Dest Y.
Viola! You have a single region common event capable of managing all simple transfers between equal-sized maps.
Bonus: Step 2 can be accomplished different ways, but the easiest for me is to set aside a special variable for DestN, DestE, DestS, and DestW. Make a parallel process on every map to assign the ID's of all adjacent maps, then erase itself. Each time you enter a new map, it'll set the destinations for the four cardinal directions. Now you have those map ID's easily accessible for the transfer. Use your character direction to assign the appropriate one to Dest M and you're all set.