I've got a cool idea for a SMW hack:
Birdo & Yoshi in: Inter-Dimentia!
The dimensional barrier between Subcon and Dinosaur Land has been breached, causing the two lands to merge. Birdo and Yoshi have to work together in order to first solve the mystery behind the strange dimensional rift, then figure out how to seal the rift before the multiverse collapses.
You play as BOTH Birdo and Yoshi (swapping between the two by pressing SELECT).
Birdo can spit eggs by default. An egg will stop after hitting an enemy or running into a solid object (i.e. a pipe or wall). Yoshi can swallow enemies/hold shells in his mouth (then spit them out). A standard jump on an enemy as either dinosaur has the effect of a Mario/Luigi spin-jump (kills the enemy instantly). A ground pound as either dinosaur is performed by pressing down after jumping. To cancel a ground pound, press up after pressing down (but still in mid-air). This can be useful if you accidentally ground-pound over a pit.
Not only do you play as both dinosaurs, powerups work differently depending on what dinosaur you are currently playing as.
If Birdo collects a fire flower, she can now spit fireballs that pass through solid objects, but are stopped by a single enemy. Yoshi also spits fireballs after grabbing a flower, but his fireballs pass through enemies (and are stopped by solid objects).
If Birdo grabs a feather, she gains the ability to swing her tail at her enemies (replacing her eggs/fireballs while the feather is active). She can also hover for short distances, much like Peach in SMB2. Yoshi grows wings after picking up a feather. He can fly for a short distance after taking to the air (flight will happen automatically; just hold the jump button). Flight and hovering will each have its use. Yoshi gains greater vertical distance while flying, but Birdo covers more horizontal distance with her hover.
When one dinosaur collects a powerup, it affects them both (for simplicity's sake--no need to add unnecessary complications).
Also, Sonic the Hedgehog-style running will be implemented (just press left or right to accelerate; no need to hold Y anymore). You won't run REALLY fast, though (I want this game to be a challenge, but not some unfair series of Kaizo traps).
Levels will be a creative mix between Super Mario Bros. 2/Super Mario Land (there are only 20 levels in SMB2 and 12 in SML), and Super Mario World (style, NOT the original SMW levels!) and SMW2: Yoshi's Island (once again, NOT the original levels!).
Graphics will be original-style SMW graphics (where possible; some custom sprites like Shyguys and, of course, Birdo and Yoshi, will be used).
Please note that this game will take me a LONG...ish time to complete (going to shoot for around five months, but may take longer depending on bugs and other annoying hangups).