In a message Russ mentioned the Ardennes theatre, so I found it. This theatre covers the site of the Battle of the Bulge, from the 16th of December 1944 to the 25th of January 1945.
During this time the frontline was really changing, and the frontline system in EAW is hardly appropriate for a situation like this.

The "frntline.dat" file has 20 frontlines 0-19. In single missions 1940 uses 0, 1943 uses 0, 1944 uses 8 and 1945 uses 12.

A while ago I programmed UAW160 to read a "Frontline.mpf" file to vary this. I have just used one which reads
9
11
13
17
These are the respective frontlines for 1940, 43, 44 and 45, (replacing the 0, 0, 8 and 12)
I selected each one using the frontline editor to which I added additional code in order to do this, and save the file

The next problem is that if I fly a P47D then the earliest year is 1944 so I either get 13 or 17.
However, there is a UAW160 fix for that too
If the root folder or the theatre folder contains a file named "NoYear" then the year values for the aircraft are ignored.
When it is present I can fly a P47D in 1940 with frontline #9, or in 1943 with #11, or 1944 with #13 or 1945 with #17.

I can also make the four years display as "Phase 1", "Phase 2", "Phase 3", or "Phase 4".


















This is not perfect, but now gives us the possibility of having four different front lines covering a short time period, such as the Battle of the Bulge

Actually, this has a lot of possibilities with other scenarios as well.

For instance, the 1st campaign in SPAW covers the Guadalcanal time frame but it only has one front line which bisects the area adjacent to the invasion beaches.

Using this mod you could have a number of frontlines, including that point in time when the Japanese had Henderson field surrounded and US aircraft were literally 5 minutes from their targets after taking off.

Another interesting "what-if" would hark back to my 1st target upgrade for the European scenario. I moved one of the frontlines so that the Germans occupied the very south of England, including a few airfields. Again, aircraft taking off were minutes away from engaging the enemy. ( I left it out of the later, much improved ETO2015, but I can't recall why I did that.)

FWIW, both of these suggestions would work well for MP.

How much work would be involved?

Nice job Jel. The ability to move frontline outside the clunky basic EAW mechanism, will be very useful.