Personally, I would love a military subtheme, or a theme dedicated to military altogether. Shame LEGO has a strong anti-war moral (even though half of their themes center around fighting, which makes no sense at all!), so this won't happen anyways. Would love me some M1 Abrams, A10 Warthogs/F22 Raptors, UH60 Blackhawks, maybe even a destroyer or aircraft carrier and figures for each respective branch. That's just me though.
As for returning subthemes, construction would be great! I don't think trains will return just because they've been releasing unannounced sets as of late (red cargo train, etc.), and to be honest, their train sets have been going a bit downhill lately in terms of originality and I'm hoping they don't return with trains just yet. Same with the airport, because I'm not a fan of the new big airport set...
I'd love if we got some office buildings (maybe a corporate office for OCTAN that isn't President Business'?) or other shops in the CITY style, take some ideas from the Friends line (I love that theme only for the buildings they put out lol). Restaurants, maybe like a seafood place, a fancy suit-and-tie restaurant, or a rooftop place?
And for a special city set, returning to seafood, a fisherman's wharf would be lovely. That, or a tourist pier with arcade/festival-like games on it, like Santa Monica Pier?
I doubt they would do any of this though. Forgive my negativity towards LEGO but they don't seem to enjoy set suggestions if it's not through the "Ideas" line, and I imagine we'll get more useless waves of police and fire above all else.
