Cities are too much of a core feature to lose. I think the game won't really be Civ anymore without them. The thing that makes this mod great is that it is a hybrid between all of these games. A hybrid, not a replacement. I think we can still have the best of both worlds here. All of the major units should still be built in cities, but there is room for lots of unique terrain improvements. Maybe only a few of the improvements could produce minor specialty units of their own. Not sure what, though, if any. I'll return to that subject after some more thought.
I do like the idea of custom tech trees and more custom units/buildings/improvements for each civ. I have a feeling that a lot of power re-balancing is going to be necessary after all of this, haha.
Since you're thinking of adding lots of new features, here are random ideas off the top of my head. I will continue to brainstorm more.
You were probably already planning to do this, but each civ should have a custom worker type.
- Workers of ALL races should be able to defend, not just Lordaeron.
- To keep Lordaeron's Militia worthwhile, they should have a higher strength than the workers of all other races. (I'm thinking 5 strength for most races' workers. Keeping the Militia strength at 8 would be fine in that case.)
- Wisps for Night Elves (25% higher production cost; can "chop down" the same forest tile forever without consuming the forest to provide continuous production; wisp functions as a temporary improvement on the forest until assigned to a different task; provides +4 production per turn to the tile while chopping; +5 after Scientific Theory).
- Probes for Protoss (only 4 strength, but receives Survivalism 1, 2, and 3, to represent their durable regenerating shields).
- Peons for Orcs (Temporarily receives Spear Throw promotion if fortified).
- SCVs for Raynor (can work together to build improvements faster)
- SCVs for Mengsk (can not work together, but auto-heal 5 HP per turn to adjacent units; healing from multiple SCVs stacks).
- Drones for Kerrigan (very low production cost; build improvements instantly, but consumes the worker; repairing existing improvements does not consume the worker; strength 4 to discourage massing cheap drones for an attack).
- Primal Zerg, Troll, Murloc, Naga, Diablo, High Heavens, Sanctuary (???)
High Heavens will need a new civ trait. Otherwise, the more custom units that get added, the stronger they will become. If you're planning on adding as many new units as I think you are planning on adding, their additional power will become too much. Besides, with the custom tech trees, it would be hard to decide when they should have access to which unit, since High Heavens won't have all of the same techs as all of the other civs. I do not have a replacement trait suggestion because I have not played the Diablo series.
Protoss do not eat the same way that other races eat. They do not even have mouths. They should not be able to build farms. Or, if they do, it should be a different type of farm. Some say that they receive sustenance from the psionic energy of the Pylons, but the wiki says otherwise.
http://starcraft.wikia.com/wiki/Protoss#Physiology One of the novels based on the game says their skin is photosynthetic. On the other hand, the back-story in the booklet that came with the first Starcraft game described the early Protoss as a tribal society based upon group-hunting and warrior rule, before the Xel'naga altered them. One of the novels tried to explain the early hunting as being for the skins only, and not the meat, since the same novel was also trying to say they got all of their sustenance from sunlight. However, this makes no sense, since hunting only to get the skins serves no practical use to the Protoss. They don't even need them to keep warm, since their bodies are naturally resistant to temperature extremes. Therefore, I personally reject the novel as un-canonical and propose that they hunted to suck the psychic energy out of the minds of animals. How you choose to interpret all of this is up to you, though.
It would be much easier to just keep the pylons as the primary food source and not have to worry about the game mechanics for extra food from photosynthesis or bonus food for livestock. The downside to this would be that if Pylons were their only farms, they wouldn't be able to build as many as other races, because of the ban on placing pylons next to each other (to prevent a player from making a city invincible by surrounding it with pylons). If you added a game mechanic that allowed ranged units to bombard improvements to pillage them, though, the ban would become unnecessary. It doesn't seem right for archers to be able to disable a pylon in one shot, but I don't like the idea of giving improvements HP, either. It's strange thematically, but is balanced for game mechanics. Pylons are still worthwhile obstacles to build if they force attackers to damage them before being able to pass through.