Either they should use FF1's world map or they should create an original one. FF1 is the only appropriate game in the series to take that from (why should Dissidia happen in a different Final Fantasy world).
If they were to blend all of the maps in the series, the map they'd end up with would be distorted beyond recognition. Though I would like the idea of creating an original world map that simply incorporated small parts of the world maps from the main series games - for example, the Floating Continent from FFIII, the Underworld from FFIV, the Big Bridge from FFV, the Serpent Trench from FFVI, etc. I think *that* could work, with a little effort.