I guess you mean with "topdown shooter", the looks of for example the old GTA (1 & 2) parts? Or do you want to do it Doom-style, thus "3D"?
In case of first-mentioned, try out
Game Maker. But be aware that you cannot make a mod then, it is a simplified game design and programming (drag-and-drop) program. So in that case you will have to make the game yourself (from scratch).