I opened Game maker up a year ago, sat down and did what I could, ended up with a very tiny, but functioning top down tank game, which wth I can't seem to find on my computer atm. It was very simple though, nothing worth really talking about, and it had a damn glitch/bug I didn't account for, driving tank into a wall and then turning would result in the tank sprite overlapping the wall sprite, which resulted in the tank getting stuck, lovely.
But anyways. A game idea could be like,
Reverse Mario. Instead of being the hero going from castle to castle trying to rescue a princess, you are the big bad. Big bad goes to the princess's castle, fighting her guards along the way there (the opening levels), fight your way inside the castle, but then, when you get to the end and defeat her royal guards it turns out she's in another castle, you see, one of the neighboring kingdoms princesses was having a birthday party, so she's over there. Darn. So, you, the big bad, fight through the next for locations/areas/levels and get to that castle, only to find that they went out for the party, like bowling or something, wtf? So then, the next level/stage is pounding through some more beautiful scenery as you go in search of the princess at the new location given. But then, when you get there, they already left and started going home. So you return to the first castle, but she didn't show up, and instead got kidnapped by another bad guy. Sigh. So, next, is go to the bad guy and get the princess for yourself. Course when you get there the hero has already saved her and they've left, so, now you chase them. And it could keep going for a few more levels like that.
Gameplay wise it could have elements of many different games. Like Mario platformer, or a beat em up, a RPG, change the setting and turn it into a side scrolling shooter. What ev.