Yeah, would be must more interesting with a mega server. One thing is that there will be more people to play with.
It's pretty hard to enjoy Mabinogi as a casual scrub because I'm not active enough to join decent guilds, and any casual guilds die within months (would feel bad if I contributed to the death of a guild so I prefer not to join one.) And without a guild, I have to run with random peeps which is sometimes fun, but other times annoying (and takes to long to run and find a party). With the advent of the mega 20 channel server, finding decent people to run stuff with would be significantly easier.
I would argue is that the character who should get to keep their IGNs are those with the highest level. My main reason is that if someone signed up years ago and no longer plays, what's the point of giving them a IGN that someone else who still does, wants? People who are still active will most likely be of a much higher level due to the current state of leveling in Mabi.
If anything, just make it into two severs instead of four; that's still a lot better than having 2 active and 2 near-dead servers.
Anyways, regarding the OP of this thread, it would be pretty interesting to see it implemented, even if it is unlikely. I wouldn't be surprised if everyone moved out of Tarlach or something. Since pets are useless without a character on a server, people should be given the option to transfer their pets along with their character.