This is my first real hack, and it's a demo. It's about 11 levels long.
Story - Bowser got sick and tired of always losing to the Mario brothers, so he decided not to try to attack them head on, but be a little more cunning...
Mario was snoring his butt off in his house when Luigi came and woke him up. Luigi had been out on his early morning fishing session, and he caught a message in a bottle from Yoshi. The message was dated a week ago and was short and slightly illegible. "
Mario and Luigi! Pleas-, you must help us. Bo-s-r discovered our native Hea--land and he and his army invaded. Most Yoshis are now dead, or m-ssing. You are our last hope. Please, you --ve to come. There is a map on the other side."
Always ones to lend a helping hand, our heroes packed their bags, brushed their teeth and swam off, following the map. Unfortunately, Mario hadn't accounted for paper's inability to keep in water, so the map turned into a soggy pulp in the mouth of a passing cheep-cheep. When the Mario brothers alighted on Yoshi Peninsula, they discovered - to their horror - that the map was gone. Due to this one blunder, the Mario bros. would be stuck in foreign lands for a looooong time.
With nothing else to do, the Mario brothers went to Yoshi's house to regain their sense of direction and then save all the Yoshis. Thus, the epic quest was started!!
And now...

The starting map. Most of the exits are revealed, and you might notice that the levels have unique names, not "Yoshi Peninsula 2" or something.

Yoshi grounds. Here, I did some custom pallete work to make the hills properly coloured.

The second part of Yoshi crossing. Lots of koopas here.

Another part of Yoshi crossing. This level is kinda long.

Yoshi gorge. This level is the only Yoshi Peninsula level to have two exits.

Pride castle. I named the koopalings after the seven deadly sins, because the Yoshis would interpret Bowser as the devil. Therefore, his sons could be manifestations of those sins.
Moving onto Yellowstone rocks, the area past the peninsula!
You'll notice that the ground seems lighter. This is because the whole area has custom palletes.

Yellowstone Heights, the second level of Yellowstone rocks. Here's a hint... Try to do the level after you've finished it once.

Yellowstone Pass, the level that you reach after the secret exit of Yellowstone Heights. Wait, what are these grey blocks? Chance blocks!
You don't know what ane does until you bump into one. They can be coins, clouds, solid blocks...

An underground area of Yellowstone Pass. This is getting to be quite a long level too!

Hey, a bridge! With big logs! This is my favourite area of my hack so far, since I made the logs' ExGFX myself. Yet another part of Yellowstone pass.
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Newer screenshots

Yoshi forest. The first level. (outdated screen)

The updated screenshot of Yoshi forest. Now it looks like there are leaves on the ground.

Yellowstone Cliff. The first yellowstone level, and probably the longest level in this demo.

A recap on some previous events.

The Grass Green switch palace. This is the first palace that you can encounter, but it's optional like the 3 other ones. I made the foreground diamonds because it's easier to use, and it looks better.

The Cliff Yellow switch palace. Non-linear, but still leads to one way foward.
Well, that's it for all the screenies. After I finish Yellowstone rocks, I'll work on ExGFX for the first and second worlds.
Feel free to tell me any glitches or things I should fix. Also, give me feedback too. Is a level too hard? Have a suggestion? If so, don't hesitate to say.