With some fairly heavy influence from one of my favourite console games ever - Shadowrun on the Sega Mega Drive, I've got about 2/3rds of the way through drawing building icons.
I still need to add some variation, internal corners, and of course doors. I also plan on having a few different tiles for each wall section (to break it up a little) and have maybe one or two more roof tiles since they're pretty gridtastic at the moment (although to be fair it's just a brown square with some browner lines drawn horizontally across).
I want the general graphical style of the new TitanNet to be much gritter and run-down than the cartoony older versions. I also plan on having two sets of object images - one for stuff on the floor, one for stuff in inventory - so that players can get a good look at their stuff.
Still a long way to go, but I'm getting there.
[EDIT] Added version with roof shadow - I'm actually surprised at how much better this makes the image look, so thanks DarkCampainger!
[EDIT2] A more interestingly shaped building.