I thing Fleksy or SwiftKey have this feature — if you swipe spacebar horizontally, it switches the language. Totally awesome. I'd pay an additional few bucks just for that feature alone. It just takes tooooooo long to hold the spacebar and them tap the language. We love speed, right?
And yeah, emojis would be a nice addition. But what's more important — I use question mark (?) a lot. And it's not on the main screen. Can you make "." and "," tap-and-hold for "?" and "!"?