Heroes of Might & Magic III: Horn of the Abyss

Will there be new neutral monsters? And how many?

in small numbers - since neutral monsters are not often used in the game. Ideally, the fewer, the better.
Therefore, the hobbit, initially a neutral monster, will become an alternative first-level unit in the Tower, changing its appearance and receiving an upgrade.

Will there be terrain variations? For example, mountains or lowlands?

like in the fourth game? No. This is not planned for now.

Will it be possible to upgrade dwellings?

like mithril in WoG?