Well, there was that Giant minicon that turned into a submarine...
There are a ton of alt-modes that I would love to see.
-Farm equipment (imagine Omega Supreme done as a barn/grain silo w/ a tractor on tracks or something). Some smaller farm equipment as minicons or whatnot.
-Pirate ship: We got a cool pirate ship from Webdiver that turned from a pirate ship into a sea monster...I wouldn't mind seeing a new version of this, perhaps as Cannonball's ride
-More city vehicles. I've wanted to see a Metrobots combiner for years. A city/school bus, mail/UPS truck, garbage truck, street sweeper/zamboni, and a taxi. Could also fit a bullet train in there too, maybe.
-More housewares, & gadgets. Some of the Real Gear figures were great, as was Ejector, but I think some of these altmodes could really shine if given a scout or deluxe treatment. We REALLY need a refrigerator TF, and Dewbot goes without saying.
I'll also echo everyone's sentiments for more World War/19th century altmodes. I'm still a little sore that Classics Astrotrain lost his steam locomotive alt mode to an update :/
-BT