I got bored so i started programming an engine for open world RPGs. (In c++ with openGL 2). The world is split into "cells"/"chunks" which are loaded/removed as you move around. Alpha-mapping is used to shade different ground textures onto the ground mesh. (the ground is a mesh, not a hightmap, to allow for more freedom.) I am also programming some sort of "construction set", which currently allows you to edit some values of the cell, adding/removing/manipulating prefabricated objects in the cell, and a utility to paint textures onto the ground.

Prefabs will range from things like rocks on the ground, to entire dungeons.

Progress:
Terrain rendering / resource management- mostly done.
Prefab rendering/manipulation - started
loading .ini files - need to start
construction set - lots of tedious work needs to still be done, but its not necessarily hard;
proper saveing/loading of cell data.
Resource manager for materials/textures/prefabs - started;
lighting - light is magical right?
physics - some progress, mostly on hold for now.

things that need to be done in feature that i haven't thought about yet:
Entity system.
Actual RPG elements like questing, etc.
Event Scripts.
Mesh editor (?!)

Pictures:

This is how the construction set looks so far: The ground repeats like that becomes i am too lazy to make proper terrain, so i just made it repeat.

Texturing the ground: (In this case removing textures.):

Added a prefab (that pink thing) to the cell: (as well as some texturing to the alpha map).