I am thinking of my a small 2d platform game using stencyl or game maker. I am aiming to make it within 3-4 days. Just trying to make a small challenge. Probably put it unto the web or a flash game site.

The story within the game would be a evil cat scientist/ evil guy with a cat who aims to make the world back into the egyptian times so that cats can become man's best friend once again.

The dog the main player in the game aims to stop the cat from this aim, but has to rescue his girlfriend from them.

Would have around 3-4 levels consisting of worlds like city map, forest, beach, etc. At the end of each level, you face the evil cats menions.

The dog would be anthropomorphic. Similar to Sonic. Like a Hip hop dog. Haven't thought much in depth about the gameplay mechanics. Jump mechanic would be the basic one. Maybe roll into a ball and attack enemies. Can punch them and can use his ears to glide in the air for a while.

Main goals within levels is from point a to b avoiding obstacles like spikes, falling blocks, scrolling background in some of the levels.

Enemies can be killed by jumping on their head. Different types of enemies: ground animals, flying animals, robot

Thats way too many assets for a 2-3 day game. Make a freefall game instead (parachute sort of thing), avoiding birds and stuff. Then you need to release the parachute in a timely manner.

Ok cool yeah that sounds like a good game idea a freefall game. I'll probably research games that have done something similar.

Why does the dog need to rescue his girlfriend? I think wanting to remain "man's best friend" is a much more interesting motivation and doesn't give in to the damsel-in-distress trope that is widely seen as sexist.

Why does the dog need to rescue his girlfriend?

I'm not sure of it yep. Probably cliche of a platform game story saving a woman. Yeah wanting to remain man's best friend sounds like a better plot story. Cats were popular in the egyptian times.

Make a freefall game instead (parachute sort of thing), avoiding birds and stuff

http://www.addictinggames.com/action-games/falldown2.jsp I like this game. It's like a free falliing game, but you have to keep the ball off the top of the screen

If you know what you're doing with those game-makers, you can get a lot done in no time at all, so I don't see what's wrong with your original idea, with a little modification to cut production time :

Leave out the last couple of lines, and use only one enemy type - a kitten which flies (or is fired? The cat is evil, after all) in one of four directions, and replaces falling blocks, spikes, etc.

See how you get on with making the levels, and if you have time, add end-of-level mini-boss cats with slightly more complex AI.

If you still have time, add an end-game freaky-Egyptian-mega-cat boss (with tentacle whiskers) with a special attack.

Fighting an evil cat who is trying to replace man's best friend sounds like fun.

Leave out the last couple of lines, and use only one enemy type - a kitten which flies (or is fired? The cat is evil, after all) in one of four directions, and replaces falling blocks, spikes, etc.

One enemy class? So multiple kitten enemies in each of the level that flies in one of four directions? But I think there should be obstacles too not difficult to make falling blocks and spikes.

See how you get on with making the levels, and if you have time, add end-of-level mini-boss cats with slightly more complex AI.

I might look at the super mario bros 3 and super mario bros wii game play for the minibosses

If you still have time, add an end-game freaky-Egyptian-mega-cat boss (with tentacle whiskers) with a special attack.

Yeah could be an temple level the last level.

Fighting an evil cat who is trying to replace man's best friend sounds like fun.

Yeah I remember the cats vs dogs film where the cats are evil and the dogs are trying to save the day.