HoTT also has a sensible treatment of both magic users and dragons. Dragons are indeed powerful, but being somewhat long lived, very disinclined to stick around when things get dicey.
As far as possible I’d try and keep things within the boundaries of the OHW mechanisms in terms of movement and combat effects or you risk seriously unbalancing things. I’d be particularly wary of modifying the terrain, as the terrain/objective interactions are what make the scenarios work.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke