I am working on a handful of different gameplay ideas.
Not sure which one I find the most interesting.
Given the time I want to make a simple demo of each and then go from there.
So here they are:

Roguelike fighter	Bullet Hell vs shooter/fighter	Swarm/Minion vs Beat em Up	Traditional Fighting game
Players get "10" fighters	4 buttons: Sword, Shield, Bullet, Jump/Dash	Create minions to beat up the oppenent	1 on 1 and tag battle
Play one at a time: when one dies they next jumps in IMMEDIATELY	rock paper scissors type gameplay: Bullet beats sword, sword beats shield, shield beats bullet. of course there can be exceptions	Fight off evil minions: crowd based attacks and combos	focus on combos and tag combos
ultra violent: "a fatality every 6 seconds"	Free Flying: think bangaio	Sorta like a moba but cooler	controls more like tekken than street fighter
Kill all "10" of the oppenents characters to win	Open space with minimal obstacles	Attack the other players minion generator and destroy it to win	basic sequences, special moves, tags etc
Each new character is stronger/more complicated than the last: first uses only 2 buttons, next uses 3, next uses 4 etc	Simple controls and actions, TONS of bullets and dodging, boosting in to attack etc.
Some characters are duplicated, except for the last character, who is extremely powerful and unique	Range changes the how the game plays and the character art, up close you can do combos even with bullets, but far "ship mode" engages, and its more like a traditional bullet hell. weird idea but sounds cool: not really neccessary but cool.

I am trying to simplify each idea down to it's core, and make them more distinct from one another.
Will add sketches and stuff that will help explain each one soon.
Currently these are themeless ideas, and there are many directions that I could take the art design -----> altho I already have some designs drawn.
All of these ideas will most certainly be 2D, pixel art perhaps.
With maybe a bit of simple 3D thrown in.

Also trying to figure out what I want to program the game in.
I have a little over 2 years experience using Unity, and I like it a lot.
Also been reading the Game Maker helpfile, but I've never really used it to make anything.
As such I'm leaning more towards Unity right now.