If you hats are going to be added, I hope they will need to be unlocked. Same goes for the eventual skins.
More than those, however, I'd like to see a couple more different backgrounds and/or level graphics.
I wouldn't know about upgrades. I think this simple a concept doesn't really need anything added to it. Perhaps something could be done with the music, as the loop gets kinda boring after a while.
Anyway, this is quite possibly the prettiest flash game I have seen in quite some time, and the controls are perfect.
Nice game really. It keeps giving new stuff to do at a decent pace.
It has some annoying glitches though. The worst one to me was the fact that often when i went to click on the select tool button to close a building tab it would build one more thing, hidden from view behind the button bar. Moreover the data for the people (either workers, janitors or whatever) is often incorrect, as it would appear that, for example, all my janitors and IT guys were hired on day 0.
I was probably lucky in not suffering from any game-killing bug, as it were the case for other people.
It could do with a tutorial, but the interface and controls are pretty straightforward anyway, and the initial goals give a good idea on where to start.
The one thing I felt it was lacking was some sort of HR menu, to make all workers' data accessible in one place, and where I could choose each one of them for evaluation.
All in all, I think this game could do with some improvementts, but shows a good potential.