In Heroes 3, there is an artifact - the Legion Statue (provides a troop increase - consists of 5 parts). Is it possible to implement this artifact in this mod, or is it possible to make the Troop Gathering skill separate and upgradeable?
Yes, this is already implemented. Here is a picture of all artifacts of the Legion set. For balancing purposes, the artifact power is based on knowledge.
Are new cities, monsters, artifacts, buildings, and magic possible?
Artifacts - MMH55 already introduces about 30 new artifacts, so yes, it is possible to add even more.
Cities, monsters, buildings, and magics are currently not possible.