Although there is no official statement or confirmed info on this as yet, apparently a product description at NewEgg.com was said to imply that Mario Kart 8 will be the first title in the Mario Kart series to offer not only Kart/Bike customisation but also a track editor.
I have said a few times in our Google+ discussions that this really should be a MUST for the title... GTA has it, why shouldn't Mario Kart?
NewEgg.com has since removed this from their product information, most probably because Nintendo didn't want people to know yet - they usually quite a strict pre-release teaser schedule it seems especially for titles of this magnitude, so I hope they aren't too angry with NewEgg for ruining the surprise.
This is still technically a rumour, but fingers crossed it materialises; additionally cool would be if you could share and download other peoples tracks online. What do you guys think? any crazy track ideas?