I'd like to know what (if at all) is exactly the engine's limitation?
You can't enter character creation again, because that's a new level? =>
That's the problem, fancy and easy example 1) tell that you (GM) your friend Bob, Peter and Morris start playing your amazing "Shadows Of Brackenborough" campaign. Bob rolls an rogue elf, Peter a cleric dwarf, and Morris an Undead warlock. You start playing with them, weeks go by, but then, another friend (Brian) buys D:OS2 and wants in, the groups has passed maps scenes and levels, enemies and stuff, you load your savegame and you have to let control over the characters to the players. Which character can Brian play? Does it pop up the character creation for him or since it isn't the campaign start and a savegame we are past that point? 2) Let's say Morris can't play, so instead of popping the character creation, you have to apply control over Morris' character to Brian?That would solve Brian problem, but it won't be his character.
3)Peter's dwarf died a hero against some dragon or something. The game gives the opportunity to resurrect, but the group aren't clerics, have no gold, and/or resurrect scrolls (Or you just simply do not want to allow resurrection in your campaign) How can Peter reroll to a new character in the middle of the campaign?
multiple GM. At least one Santa's little helper GM slot which maybe hasnt full control, but can place objects / characters
You can invite people as secondary GM, I think