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I rummaged around in the editor and found some interesting objects, which I'll post here. Of course, this isn't related to Easter eggs, but after reading the thread, I realized that's not what we're talking about anymore. =)
1) These are different heroes that you won't find on the map and can't even place in the editor. They're from dialogue cutscenes. Among them, you can spot "mega godzilla," as well as a tank-hero and a UFO.
2) Portraits. I have a guess who they are. =)
3) No comment.
4) The throne room of the Dwarves from the cutscene.
5) The Castle of Haven, either from the editor or just an unfinished model.
6) Ships of the Academy (covering half the map).
7) Looks like a mausoleum. Or something like that. In general, it seems like a lot of events from the Heroes campaigns took place there.
8) Various interesting objects. The ruined walls of Haven look great.
9) Monsters. I didn't find a normal Plague Ent or Yeti, I'll post them later. But there's a Dark Knight, as well as a Fire Bird, with ready-made abilities and animations. It's a shame they didn't put it in the game, along with a building to purchase it and make it an upgrade to a Phoenix.
10) Nicholas with someone's heart. An artifact, basically.
11) Tieru. And that says it all.
12) Panzerklein. And that says it all.
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There are also various ghost heroes there – Tieru, Isabel, etc. You can search for them.
I'll try to find some interesting buildings.
By the way, there are also a lot of interesting landscape objects there – mountains, rocks, etc., which weren't included in the editor and aren't on regular maps. It's strange.
I also wanted to find a Yeti, a dead Cyclops (from Dark Messiah), and a Plague Ent. I guess I wasn't looking in the right place.