I like Maztica (and some of the other FR sub-settings) too much to move onto the 4e world. But I have heard a couple of people talking about getting the bits that were lost during the Spellplague, putting them onto a new map, and then filling up the rest of the world with stuff similar to Returned Abeir (to create an Abeir that contains all the bits of Toril that vanished). That seems like an interesting mapping project.
But if someone ever did that, I think I would like to see them put Returned Abeir back onto the world Abeir and then use the pre-Spellplage Abeir as an alternative world to Toril (during a campaign set during the 3e or earlier version of Forgotten Realms). A project like that would be the thing that would be most likely to get me to buy the Forgotten Realms Campaign Guide. If I had a fan map like that, I could use it alongside Maztica.