My project right now is developing a hexcrawl. I always thought 4e would be great for this b/c I found the encounter budget worked well as an indicator of challenge (with some tweaks). So, populating the world map would be a decision of (for example) hex A1 would have an equivalent of a L1 encounter while hex E6 would have a L5 encounter budget (with appropriate rumors and clues for adventures to find to avoid or confront). Wandering Monster tables would include a range of levels, forcing players to decide whether they should run away, if the fates are unkind. Anyway, that's the idea for now.