It's kind of ironic but I'm pretty sure you could recreate the entire campaign in GM mode and then you could have people join as whatever. I would assume the major barrier for them to making this happen is one (or both) of two things:
1) They are concerned that pub games would be ruined by over-leveled characters joining low level games. (Which is an easy fix, you just put a cap on level difference to join a game)
2) The framework/netcode they are using for the campaign is fundamentally incompatible and would require serious reworking. (Which would mean that we definitely wouldn't get it on launch and probably ever)
If they did do it though, the easiest way would simply to have the game pause for the host when a player joins and has selected the character they want to play. Then the host is presented with the faces of their NPC party and selects enough NPCs to kick to accommodate the PCs that enter the party. The NPCs poof, the PCs join, and when/if the PCs leave you are allowed to bring NPCs back into the party that appear the same way they left. The only issue with this is that there might be exploits here that would make the game trivial, but it's an rpg, if they want to ruin it for themselves then let em.
I'm pretty sure GM mode also doesn't have the ability to bring in new characters whenever, since the GM needs to save the game and it saves the existing characters for use by the same players as I gather it.
