Siege can be used from one of standart sieges (of from towns with finished towns).
Creatures can be found in packs etc.
Heroes can be made from available portrat packs.
Puzzle is one of most miserable things. On difficult maps there is no time to search for Grail.
Standart puzzles are good enough.
Screen is the most difficult part.
For example, Bastion is easy to build, there are sufficient high quality screenshots to make all dwellings. Only question are units, but there many Egypt-like units.
But there many time for testing and polishing is needed.
Even if not polish graphics, there balancing is needed.
And there not much sence in building “release” of towns, since VCMI is not 1.0 version, and format of json’s is changing often.
If town is released today, after 3-5 mouths it will need rewriting.
So now it’s better to prepare graphics for dwellings and units and make test builds.
PS By the way, to speed up puzzle creating, i think, it can be made from quadrats. And for different fractions only picture is needed.