Isometric view and perspective (position objects with only the view from the top is sheer madness).

Object hierarchy and ability to handle objects with layers

Water brush or river water plane or plane (you know that in the mountains there are lakes or rivers?) - As torque3d

Add different types of brush to create the ground (as AGFPRO)

The brush to create the grass must allow the user to select different types of grass simultaneously. (as AGFPRO)

Add the ability to select any texture to paint the ground (as torque3d)

Multiple selection of objects.

Snap height !!! (ability to edit the snap), not only 1-10-100 inches (it is a huge limit)

Emissive map - displacement map -glow - reflective map.

Particles like - ground fog, volumetric fog, smoke, lightray, dust, rain, snow, fire (the fire divided into several categories, such as: candle, torch, campfier etc etc) ... etc .... etc. etc.

Try using torque3d and take inspiration
Forgive my english