^so there are at least two "worlds" or more accurately franchises that are not revealed yet, my guess would be Pikmin for one of them but i have no idea for the other.
(and there could be more after 20, but 20 seems a nice round number)

I am guessing also that there will be a stage for each franchise, so we are yet to have F-Zero, Metroid, Mother, Ice climbers or Sonic stages yet. (though i think, like the F-Zero stage, they might have been seen in images but not announced yet.)

No F-Zero stage has been seen yet...a Mario Kart stage has though.

I can't wait for the Metroid stage...I hope they make something more original than a "Brinstar Depths" type stage again.

#13 MUST be G&W...but #19...it could be anything. Pikmin, Megaman, Castlevania...

My thoughts on Sonic:  It's pretty much like I imagined he'd fight in Smash Bros...a number of kicks and punches, and then his rolling into a ball form.  I love the "B-down" move...being able to charge up and let go...it'll undoubtedly feel just like the old days of 2D Sonic.  That said, I expected more rolling into a ball and less kicking and punching, but it probably wouldn't work as well or look as good that way.

Brinstar?

Also yes i was thinking of the Mario Kart stage (but mixing it up with the old F-Zero one which is a similar concept).... but which world (franchise) would a MarioKart stage go in, none of the things i posted have had two stages yet, but Mario kart fits best into the Mario World. (although Yoshi has a seperate world so i dunno)

 Edit, damn i always post interesting stuff at the bottom of the page....should i post it again on this page?

Only if they will say Marth is in the game now.

I was thinking that could be the lava stage from Prime, Twroo.. Some word that sounds like "Magma"... caverns.

I just wish they pick something other than lava or acid worlds...I realize it instantly associates with Metroid, but a snowy Phendrana Drifts level, ancient Chozo Ruins level, Pirate Frigate (though it seems there's plenty of "metal" themed stages already with Snake's, Fox's...)

 Remember they announced Motorcycles in Mario Kart Wii? Wait doesn't Wario have a motorcycle! Coincidence?

Sig Updated.

Added:
-Lucas
-Sonic
-Snake

Enjoy!

Original Size:

This may not necessarily be Feature Creep.  We don't know what was in the original spec document.  It's likely that all these new additions have been planned from the beginning. Every project has a certain level of feature creep, but I don't think that's what's going on here.