OK, this is an idea I've had knocking around for a little while. In case you aren't aware of what a roguelike is, it's this: http://en.wikipedia.org/wiki/Roguelike
But obviously this isn't a turn-based top-down game, so here are the key features (pasted from that page) that I'm thinking of capturing:
* Roguelike games feature randomly generated dungeon levels, which give them more replay value than games in which the levels are the same every time, though many have static levels as well. The randomly generated levels typically include rooms (some of which may be preset or specific, e.g. monster lairs, treasuries) and corridors / tunnels leading from one to another, though more open non-room spaces or other elements, like rivers, may occur.
(So, the rooms and corridors would actually be side-on rooms and landscapes, like Castlevania, Ghouls 'n Ghosts, etc. "Randomly" generated makes it sound crap, but the trick is to work out a process that "grows" the level and then decorates it and populates it with monsters, items, etc. I think I've got a decent set of rules but it'll be interesting to see how well it works)
* (MAYBE THIS ONE) Roguelike games traditionally implement permadeath—death is unrecoverable. Once a character is dead, discounting item-afforded preclusion, the player must start over from the beginning of the game. Most roguelikes provide a "save game" feature, but this is only intended to allow splitting a game across multiple sessions, and the save file will be deleted automatically when the character dies. Players can avoid this by copying the file to another location, but this is considered cheating and often known as "save scumming". Some roguelikes provide a "wizard mode" that lets players explore the dungeon without risk of death. Players may also encounter items during normal gameplay that can prevent death (usually just once).
(This may be too harsh, but one consequence is that there is a lot of tension in the game. To reward the player if they fail, each attempt allows them to learn more about the game, and each attempt should be an interesting and often funny story in itself. It's also important to allow the player to work out how risky a situation is, and then give them a choice whether to get into trouble... most of the time)
* The world of roguelikes is heavily interactive and players can often perform tasks impossible in other games, e.g., digging through walls or lighting fires.
(There is usually a lot of emergent gameplay, i.e. simple things combine to make interesting situations. Another thing that is common is the need for the character to eat, and often make risky choices in order to avoid starving)
My original plot idea was very basic, just a tongue-in-cheek plot to hang the gameplay on:
* Kings and nobles complete with each other to amass artifacts and treasure
* They pay collectors (players) to go out and bring something back - maybe anything (payment dependent on value), something of minimum value, or a specific item.
* This allows the player to return early with a low-value item but risk loss of reputation and low reward
* The patron creates a portal which stays open (i.e. the world beyond persists) until th job is completed or abandoned
* A collector might have a single patron, or play patrons off against one another to drive up prices, especially as their reputation increases
* Things collected visible accrue in the patron's castle
* High-level player might be able to build and furnish their own castle
Of course, it could be more varied, and another idea was that this could really be found by the player to be a distraction from the main plot, e.g. finding a missing friend, or the underlying secret behind this greedy society.
Anyway, there you go! I have more stuff on this, including thoughts on how to great good procedural content involving traditional authored content.