1. MMO's are not about having a time sink, but having a big playerbase, and being able to team up in large scale "events". But i do agree on what you say, building up your town should take longer.
2. Well citys are for training troops, it's a MMORTS, not a MMO-build-your-castle-and-manage-economy.
3. balance issues will get better and better as time goes. And mercs should honestly be removed. Units SHOULD die, thats where the RTS comes in, you have to use your units smart, if you dont want to loose something, dont get it, or have it in a "safe" place.
4. Due to buldings taking time to build, i find this game kinda casual as it is already. Casual players start a building, and logg off. Hardcore players start a building, spend some wealth to speed it up, and go for some PvP to get the wealth back.
5. All i can say is that i agree with you here
"Remove unit / upgrade caps"
Just no, this would make this game more of a hardcore game, instead of a skillbased game, or possibly just split up the playerbase more, as higher tech players would end up getting a even higher army strenght.
"Merc / Trainable units"
As i said above, i think that they should be removed. But your ideas about having an upkeep sounds good, aswell as them not being able to get exp.
"Increased storage capacity of main city"
i kinda like the low storage, this makes it so you have to store some resources on unsafe places. This is a PvP game afterall, nothing should be safe.
I dont even think you should be able to train them at all. Units should get stronger from battle experience.
"Send resources home"
This would just make the game "easier", if you want to get some of the resources to your town, get an escorted cart army to move home with it. If they get attacked they will, as i said before it's a pvp game.
I like the PvE ones, but ONLY if you loose the troops that you loose in the scenario.
This means, that in the wave defense scenario, you will be able to pull out troops, if you move them to a specific place on the map, so if you play smart you can save all your troops. And if you play bad, you might end up loosing some.
The first PvP one sounds good, but this should be a coop scenario tbh, where you play 2v2 or even higher.
Grouping up will, afaik be added sooner or later. Getting the pvp lag and desyncs fixed before is of more importance though.