Oddly enough... I was working on a similar concept years and years ago, as a text adventure.
The first layer was the standard text adventure fare; talk with people, interact with objects, and so forth, used in cities to buy gear or get quests, in ruins to solve puzzles, and so forth. It was purely text-adventure; I had even started to add some "long term" puzzles like investments.
The second layer was a rogue-like exploration system, moving around a (randomly generated) mini-map, discovering cities or ruins, and traveling on a macro scale (a 24-mile-hex view, as it were). It consumed resources like food and water, extended the map, encountered random monsters, and allowed you to travel between cities at a "realistic" pace, as opposed to either typing "go north" a million times, or having an eventless "you travel north for 20 miles..." every time.
The third layer was a battle system, which I must admit I put the most time towards. Single or multiple monsters, and one adventurer, though I had planned for the ability to have henchmen that would act on their own unless told to do otherwise ("tell William to attack the ogre" or "tell William to fight anything").
It was never completed, obviously, but this thread reminded me it still exists. One of the hardest parts was coming up with all the monsters, armor, damage, etc.; if I use ACKS rules, I could easily throw in all manner of creatures, attacks, etc. I'm going to have to look into that, some day. Not any time soon, mind you. But some day!