Man what was so great about Super Mario World? It's good, certainly, but ... geez.
Super Mario World didn't get a sequel ever, jdaster. Yoshi's Island is not really Super Mario World 2. Also, games used to improve on one another (see SMB->SMB2j->SMB3->SMW, although SMW didn't quite improve in a few areas) rather than "stay true" to the last one. NSMB is weird but it still has cool new stuff, and a game that just has stuff that SMW already had wouldn't be exciting.
I think Yoshi's Island is just about at the pinnacle of doing awesome stuff with old ideas, but it's been shown that Nintendo is too busy to make a good sequel to it.