I wonder if there will be any new extras ? According to
SteamDB, the GOTY edition will apparently include 5.03 GB of extras, while the
original game has 4.16 GB (art pack) + 163.77 MB (HoS and BaW extras). That adds up to less than what is in the GOTY edition, although I might have missed something, or it could be packaged differently (do the free DLCs now count as extras ?), or include stuff that was previously only on GOG. Anyway, we'll see it soon.
Even in the previous EEs, the majority of the new content was not main quests: fan made adventures in TW1, and the two new side quests in Chapter 3 of TW2. The latter did add some new cutscenes, but they are not really story changers.
The GOTY edition was made a "new" game so that it can include all the DLCs in one package, rather than as separate downloads like they will remain in the original version. But that decision means that for example on Steam the GOTY edition will have a new AppID, a new community hub, new cloud saves, achievements, and so on. And it is probably similar on the consoles, that is why the saves do not carry over; on PC, it is probably still possible to just copy the files. Perhaps it would have been possible to implement some kind of import feature, but I guess the GOTY edition is mainly for new players.