This idea is really just an extension of the 'Income Statement' screen -
I think it would be really useful to have a screen showing how many of each of the available units are present in the tower (IE Apartments: 23, Music Stores: 3, etc...), this would allow the player to better decided what to build next.. In large towers I often have a hard time remembering what I've already built (especially if I'm playing a lot of really short sessions). It would be nice, but not essential, if that same screen could show the demand or proft/loss for each unit type.
-Dan