Well, I don't know. Originality is important. Just to be clear, I think I'll explain what I mean by clones. If you made a Basketball game or a Chess Game, I wouldn't consider that a clone. If you made a game that looked like Arch Rivals or Battle Chess, I would consider that a clone. Chess is general... while Battle Chess is specific.
I don't want to see Pac Dude or Super Mario Cousins, as that's an obvious rip-off of Super Mario Bros. Think of the TYPE of game you want to make. There are plenty of side-scrolling platform games that are not clones of Super Mario Bros... like Earth Worm Jim, Sonic the Hedgehog and Alien Hominid. CastleVania: Symphony of the Night is a way different game from Super Metroid, but they're very close in format.
In other words... DON'T MAKE ARKANOID!
The first step is to realize what types of games Flash can make. High-Polygon 3D games, probably not... but Isometric games can be done in Flash. Jobe Makar has a good game design book - Macromedia Flash MX Game Design. It covers all the important things to consider when making a Flash Game, Like sound, graphics, physics and game types.
It's important that the type of game you're making matches your skill set and the amount of free time that you have. It would be awesome to make an Isometic Game, but that is heavily dependant upon graphic creation. I wanted to make an Isometric RPG in Flash, but I quickly realized how difficult it would be. If you don't have Jobe Makar's book, Kirupa.com has a great tutorials on Flash, Game Development and Isometry...
http://www.kirupa.com/developer/flash/index.htm
Here are some game types that Flash can probably handle...
- Tile Based Games: 2D RPG / 3D Isometric Game. Flash can do A* algorithm (for character movement) and title sets. Old School games were often created with graphic tiles. Flash is better with Vector graphics, but there are ways to treat vector graphics like tiled bitmap images. (IF you do an RPG, you'll have to figure out a way to add scoring.)
- Space Shooter: I don't see what's wrong with making a space shooter. While it's not too hard to find tutorials on creating a scrolling shooter, how many good Flash shooters are out there? Many of the ones that I've seen were unfinished, ugly or not very fun to play.
- Sports: There are so many possibilities here.
- Ninjas: Who doesn't enjoy a finely-crafted game about ninjas?
- Puzzles - This is tough, as I won't accept Tetris, Collapse or Bejeweled. How do you make a puzzle game that's original? If you can figure something out, Flash is generally a great platform for puzzle games.
Getting past the blank page is a tough part of game creation.