Intro: I have browsed the site, but have not seen a plan for a Arena remake with any idea how programing works.
Dealing with the Flat and 2D World: Data-mine ESO for the terrain's bump-maps (they are split into holds) and piece the bump-maps together in the Unity engine. We procedurally generate trees and bushes using perlin noise.
Texturing: we create 16 different textures for grass (or lackof) and plants. 1 plant texture per season for forest biomes, 1 plant texture per season for desert biomes, 1 grass texture per season for forest biomes, and 1 grass texture per season for desert biomes. Other textures would be static and easy to find as royalty-free.
Buildings: handcrafted interior and exterior. Caves and Dungeons (since less complex should be generated with perlin noise.