I can think of something that might be fun to add for people who like to build up - power generators. Their use would be to power a building, opening possibilities for operating refrigerators (reducing spoilage rates for perishables), electric lighting, computer consoles (that is, games for morale), and a general oasis of civilization for your home base. That is, assuming you go with a stationary structure and don't do something like use an R.V. converted into an armoured tank for a mobile home base. You could have the usual loud, noisy, zombie-magnet gas-powered ones that would require regular foraging trips to keep topped off, but also quieter electric ones based on solar power that shut down at night unless you have enough battery capacity to keep them running 24-7. You'd be able to loot these intact only rarely since, as noted, they'd be complete zombie-bait if left running in hostile areas, but if you don't get that lucky, you could also rig up one of your own with enough mechanics/fabrication skill either using car parts or from scratch. I suppose there's a question of the "feel" of the apocalypse, though.
On that note, roving threats might be somewhat interesting. Though I haven't been able to ever get very far, whether on static or dynamic, once you clear an area, it seems to remain cleared and (relatively) safe. It might be interesting, once the initial days of the cataclysm pass and turn into weeks or months, things like zombie hordes that move in search of survivors after having scoured their original grounds clear; fungal spread that creates a relentlessly-encroaching threat; young queen ants and an entourage moving to create new hives; or maybe completely new threats that we haven't encountered yet. After all, in any zombie film where they do create a stable settlement or simply a small base where they can live in relative peace before the last five minutes, the zombie threat always eventually reaches the protagonists, after all. It'd be a test of defensive building and contingency planning, as well as a new challenge, and nomadic players could simply try to move around the threat since they don't have to defend a fixed location.
Also, love the idea of feral tomatoes as part of an agricultural/plant growth overhaul. We have C.H.U.D., Them!, Day of the Triffids, and the like, so why not Attack of the Killer Tomatoes? Also, if you want plants that grow in spite of a person's best efforts, try rhubarb. ^_^