A cubeRL could be good, but I hope that you come back to this project again if you decide to set it down.
I've been trying small experiments with terrain generation lately, though I haven't gotten very far so take my advice with a grain of salt.
That thread I linked earlier contained a quote I got from
The Making of DF about world generation. Although creating something the scale of the DF world is unnecessary for this project, there's still some relevant ideas.
Creating an overhead map, whether you ever displayed it to the player or not, could help with distinguishing the terrain. Each cell of the map could translate to an X by X region in the game, inheriting all kinds of information to make it a specific and unique region. Starting with a
heightmap doesn't mean that you have to display a z-axis, but it might help determine where the treeline is, where the swamps, rivers and lakes are, the temperature, etc.
Once a region is well defined, you can fill it up with the appropriate plant and animal life, at the appropriate density, with some
perlin noise or whatever.