Suggestions to fix battlegrounds:

Bgs is the content pvpers were promised as they had nothing for a while since ic, The problem with bgs is that the population isn't big enough to handle premades and separate cp, non cp battleground ques and there is no incentive to run bgs unless you are looking for bg dyes/achievements as you can get just about the same and even more ap the same amount of time in cyrodil as it takes to complete a bg match. My solutions

Make Battlegrounds Base game: This will alleviate the population issues that battlegrounds have and you are far less likely to run into a premade group as you will have a bigger pool of people to grab from. Its a constant issue if you are a solo player and it turns off people from battlegrounds completely if a organized premade completely demolishes you. Bgs in their current state are not friendly to new players.

Create a Battleground Exclusive Currency: Like tel var, give a reason to go into BGs and grab exclusive rewards. There is no real point to running bgs as all the rewards are currently purchacable for ap. You can just farm in cyrodil and learn Militant Ordinator and get battleground exclusive gear with all the ap you have hoarded. Ap is in abundance and these items are easily obtainable compared to if bgs had launched with a currency. Keep Militant Ordinator and gear as ap buyable but introduce new rewards

New Rewards for Battleground Currency: New motifs based around the Storm lords, Fire Drakes, and Pit demons, as well as Furnishings, mounts, pets, and other goods to be purchasable for Battleground Currency.

And if needed... Lock the Merchant to morrowind/summerset, or chapter locations: Personally I am against this but zos wants to keep bgs chapter exclusive, opening up battlegrounds for base game to help alleviate the problem will give a larger population access to battlegrounds, but locking the merchant access to morrowind and chapter locations would give incentive to having the chapter.

Suggestions to fix imperial city:

Separate Imperial city from Cyrodil: Imperial city does not contribute to cyrodil and people in imperial city can take away player slots of population towards people fighting to the alliance war.

Add campaigns for the imperial city: Both cp and no cp, And quing in should let you teleport into the sewers immediately instead of having to travel through inconvenient horseback to actually access the dlc.

Add A pvp Asylum/master/VMA type weapon set to pvpers: Pvers get all the fun for weapons and forces pvpers to pvp in order to get some asylum/master/vma weapons, add in a new pvp weapon set exclusive to imperial city to help breathe life into it and give incentive to run imperial city