I am a solo developer, and I've been working on a mobile top down shooter. I've been struggling to come up with an appealing art style to use, but I'm leaning towards a cartoony, flat/cell shaded art style with thick black outlines. I want it to be kind of gritty and messy though (like rough, mouse drawn outlines and shading). The problem is, I can't find this style anywhere else, so I can only go off of the very vague idea in my head (very hard to describe or actually create). The YouTube channel BlackThornProd uses an art style somewhat similar to this. Does anyone know of any games with this art style? Any other thoughts?

My second problem is trying to come up with generic map ideas. I want the game to have both futuristic / modern / medieval weapons, but I'm not sure how to include all this without everything feeling out of place. I don't want the maps to lean too far in a certain direction either. Like I could just have a desert map, but how do I make it interesting without any man made structures. I'm not sure of many games that don't have a setting like this, so its hard to come up with ideas. There is BTD5, but those maps couldn't apply to my game. I was also looking at TF2 and Overwatch, but none of these games have what I'm looking for. 

I would love to here any ideas or thoughts you have! Should I scrap all my ideas ;) ? Thanks for your help!