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So I sat down to try and think of an idea for a Simulation/Role-Playing game, and I drew a blank. I would really appreciate some ideas...

Well I guess it's about making a multiplayer game, right? Decide if you're up for making something that fits your dream or the genre/storyline you'd like to play yourself, or if you're going after more commercial topics. See what kind of special options/characteristics you like in games anyway: is it the accent on micro-managing in simulations, or close-zoom-with-each-unit-important strategies, or fantasy storyline that lets you progress by solving puzzles or finishing quests, or maybe you enjoy large and unique maps for exploration, etc. Also, what seems like a good formal thinking to me is to decide on the future of the game itself - thus thinking out the main mechanics/engine of it: particularly in a sense of how do you want to add updates as the game progresses. For example, strategies should be made in a way that allows for new troops to be added later, fantasy games should have room for new creatures and/or quests, simulations might be open for new events ingame, or card games especially must have engine that can easily take new cards. Last but not least, what would you like to be your games' "wow" effect? Will the players awe at the graphics, storyline, detailed list of items of interest, careful planning that's asked from them, etc?

I'm always more interested in those formal aspects of the games. and combining the answers you get the idea of what language/platform to use, where to put an accent in the early stages of development (mechanics of the game-events resolutions, graphics, storyline, OOP logic that allows for changes later, etc.) and how to put new ideas into it later. Thus, draw/write/choose the 'framework' of your game, then see about the genre - or if you come across unique genre or storyline the framework might unwrap itself on it's own

As for genre/storyline itself, I have few ideas to share on that formal level too (not like suggestions though):

1. Zombie games: even if popular, there are just too many of them and it's hard to get dedicated players. The 'wow' effect here is needed the most: something that other games don't have. I'm not 100% sure about it, but it seems to me that a 'pocket' of options here lies in the micro-realm: close-up of zombies, your character, focusing on details like weather, part of the day, specific wounds you can get (like broken finger... why don't these games have such things? xD) etc. I think most zombie games lack that kind of details. Is it to hard too make? Well, that's another topic xD
2. Fantasy games: even though I personally dislike the genre, this is probably the vastest realm of ideas and there can surely be something that can suit you. Just make sure the 'wow' effect is even more accentuated than in Zombie games - roleplaying Elves all over again seems kinda boring to me xD It might be the graphics or the quests details themselves, but you need something catchy and out-of-the-box. Maybe putting in some twist that draws from other genres/topics, like politics - Dwarves might be presented as communists and who knows what Elves might be... blonde, tall, I'm-better-than-thou fascists? lol
3. Sci-fi games: those are catchy since players usually expect it to be 'regular' in some way (to have trade options, combat options, a bit of micro-managing etc.) so framework is a bit revealed on it's own. You'd need your own 'watermark' too, it might be (like in that new game of which I forgot the name!) the detailed interior of the starship, with each crew member actually operating separate tech etc. or again some flirting with other genres: Dwarfs as the space miners; maybe even total and utterly weird accent on weapon details, where players have to work out each technology and combine them to build 'custom' weapons.
4. Crime games: lame. The players should go out and do it instead of playing them to look tough.

Just some ideas out of my head.

What's your ability to code?

Might just want to make something simple, something really easily within your coding abilities. Then just throw things together. Kitchen sink it - you don't need one co-hesive idea - it doesn't just have to be 'zombies', it could be mining for ore on foreign planet for ore that have space zombies, while you negotiate with cyborg vampires.

Don't bother with a complete theme - just think of the individual creatures/people that seem cool to you. Just have them. Then the rest of the world will change itself to make it fit around these creatures and people exist.

I agree. Cohesive themes and game worlds are really only required for AAA commercial titles that are trying to draw you into a believable world. Some of the most famous independent games had strange ideas and worlds that were not entirely cohesive, or even were totally absurd. It can even help your game to stick out