The great rail canon was discussed here (someone even created a model on civ5) and could be a cool Wonder unit. I like the fact it is bounded to railroads, making it more strategic.
Otherwise, the SS Great Eastern could be a nice Industrial Era Wonder (Don't know what we could do with it however.. can be relocated in different cities? Extra trade route?)
Speaking about the mountains, I hope wonders relative to them will be back. I loved Macchu Picchu and Neuschwanstein Castle, but with the one-Wonder-per-tile system, I'm afraid it will be complicated (no use for Chinese ability since units cannot cross mountains, etc.)
Neuschwanstein is amazing, same with Macchu Piccu but i was always fond of >4 city builds. macchu Piccu really came alive in my India game, something like 800gpt from just internal city trade without any other city trade boosts.
We could possibly get 1 Wonder Unit for each era, not from the dev's of course(.01% chance, maybe

) but as Mod it might be able to work, trick would be to somehow make a unit that behaved like a wonder..

...You could always create a new Blank Wonder that spawned a unit.... thatd'd work

... except now Wonders need tiles
So we have :
Trojan Horse as Classical,
Schwerer Gustav for Modern,
SS Great Eastern for Industrial.
.... What else we got?, a quick hunt thru google
brought us the Da vinci Gatling Gun, could work.
Ancient and Middle Ages i have got no idea, we would also need a badass Atomic and Information Era one as well. anyone think of anything feel free to share
