Hello (sorry for late reply, I was in a holiday by the way)
Yeah, Engineers can already capture buildings but we haven't thought of Spies or Infiltrators yet. (but we have Mind Control system done, only problem is AI can't use them automatically). Tesla technology will definitely be included, and they are perhaps one of the most simple yet awesome stuffs to add. (all lightning sounds will use RA2 ones, they sound the coolest)
Simple Individual Chrono Teleport (Chrono Legionnaire, Phalanx, etc.) is easy to add, however Chronosphere Teleport requires massive amounts of complex coding to add, especially RA2/3-ish Chronosphere.
Do you plan to make units recieve veterancy levels? It would be great of course, but I can't imagine how it can be realized and if, how much work would be spent on creating this mechanic, maybe better to leave it for now?
Yes, but let's leave it for now.
Global powers for every faction or just the C&C ones?
A bit of both, but no Horde (though they will appear in the first few missions where they will all be eliminated):
Alliance : Tridemist China (ROC) + C&C and Mental Omega Allies (USA, Euro Alliance, Pacific Front) + Azerothian Alliance (Stormwind, Khaz Aerie, Night Elves, Silver Covenant, etc.)
Comintern: Soviet Union + Red China (PRC) + Communist Quel'thalas (and Elvenhood of Flardyrn, a secret society back then)
Epsilon: Yuri is Master!!!
Illidari: You are not prepared for Illidan and Vashj! And Kael too!
Secret Faction: Rule of thumb, players! You can't can kill the messiah!!!
I can imagine a button on a Town Hall, when pressed would show some powers with a long cooldown. Maybe a triggers can be made, counting kills and unlocking those powers..
IMKO own Hall is too simple to include fancy stuffs. Don't worry however, as the Keep and Castle has some wonderful magics.
I'm not sure how you feel about resources; there are Gold, Lumber, Ore, Tiberium.. Seems overwhelming.
Mineral: Gold, Ore, (maybe half of) Tiberium Crystals and other Harvestable Raw Metals
Lumber: No idea. I plan to have the trees unable to be harvested, they'll most likely be just simple decorations and obstacles (just like in C&C games). Can be destroyed by certain weapons.
Even if Tiberium isn't considered as a separate resource, it's still deadly as ever. I may consider separating every half of harvested Tiberium Crystals into Mineral and Tiberium (remember, a Tiberium Crystal also leeches other minerals to the soil). If you harvest 5000 worth of raw Tiberium Crystals, you'll get 2500 Mineral and 2500 Tiberium.
I'm trying to imagine high-tech armies suddenly appearing in, let's say, Ashenvale forrest. Will they establish some kind of city and have an industry to produce their stuff?
What if Undead appear in Washington and start rampaging, later fighting the police, army etc; how will they fuel their war efforts if there is no gold available to mine? And what about other races..
First, a messy civil war happens in China. Tridemist China and Red China waging war on each other. Then all of a sudden, some rifts open and suck some of the folks from both sides. They then appear in Azeroth -- the Kuomintang (Tridemist China's ruling party) and a few battalions from ROC Army in Elwynn Forest, Stormwind; The Communists as well as some Soviets find themselves in Quel'thalas......
If you don't mind, can you give some ideas on MegaCraft (story, units, etc./ and you'll be credited)? You can also join the Flames of War roleplay if you're looking for some fun together (but you'll need to join RACC social group first)