Ideally it wouldn't have to be modded in, it should be something the game supports natively. Lot sizes should be far bigger than they are now. They're tiny. People shouldn't have to rely on ploppable items to build up their city like a model village.
This was one of the things that hamstrung SimCity 4 even though it had much larger lots, and as a result it showed the absolute best talent of the modding community when it came to building ploppables, and ploppables with functional utility.
Along with bigger lot sizes for buildings, it should be possible for people to link their assets together in the editor or in a settings file for their asset, and have them automagically expand into neighbouring lots, choosing other linked assets when they do so. For example, a shipping business buying out nearby lots for extra cargo space, farms buying lots for fields, etc etc.
I'm surprised and kind of disappointed that it doesn't happen already.... I feel like I have to prefix comments like this with 'this game is great fun to play, a great achievement for a small team etc etc, BUT...'.
All that is true, but some small things like lot sizes are really frustrating. Especially the tiny sizes given over to farms. Farmland just doesn't look like farmland, and doesn't function anything like farmland. Industry is really the zone to suffer the most, since it really wants to be claiming nearby lots to expand out, as a process of doing big business. The flow of the industry in your town and managing it should be one of the compelling gameplay forces. It's sort of there. But without business owning multiple lots, not really.
I really hope that it gets improved upon given the massive success of the title and the obvious demand that the modding community has. Even as a basic first step, I would just like to have unlimited or very large lot sizes made possible, or some kind of support for modular lots.