My advice is if you want to write a game you should creat one that you will want to play.
Why? Because you will most likely be the only one playing it the most
The most popular roguelikes are ones where you descend a dungeon, pick something up from the bottom and go back to the top. Any variations on this is not really needed but just add flavour, e.g. branches, (random) overworld, multiple dungeons, multi-party, towns and real-time *shudder*.
I think all you really need to come up with is a theme, everything else is will come naturally.
Now ignoring everything I said, because this is what you really are asking, how about a roguelike based on post-apoc gunslinger/mutant/cyborg/psych game. Awesome. Gunslinger are weak but kill from afar, mutants have all sorts of skills and mutations to get them far, cyborgs are strong but cannot heal, constantly needing repair/upgrades, and psychics are you magic class. Not get coding
Now you need a random overworld, mostly desert with few towns, oasis, caves etc.
Most people are hostile because strangers are not to be trusted and resources are scarce. You walk around trying to befriend towns by doing quests, teaming up with the occasional NPC. Of course you need a goal for the end game...
Now get coding