City Sacking: PvE vs PvP server guards are currently VASTLY different. PvE realm guards are harder currently. I would like to see blizz revert all guards to be easier.
Opening City Vault: An excellent idea. Requiring a rogue-engineer is to specific however. WoW has turned into a game that works more on class equality, and on a super small pop server, an engineer-rogue might not even exist for days at a time. Opening the vault should require 3-5 players doing a channel, or something such as that, not something super specific.
The loot: Great idea, until you started talking about pulling gold and items from top people. This might be semi-but-barely ok on PvP servers, but wont work on PvE servers where no one is required to PvP. The entire idea of pulling loot from others would not pass. Lets just keep the loot / gold generated as random grey high gold vendor trash, and rare boe / bop toys and fun items.
Fencing and such: This would actually be better as an independent idea, unrelated to the above capital city vault. The fencing items could be obtained from smaller "Vaults" that are randomly chosen day to day.
-Each day a random smaller city will be marked with a vault symbol, this city can be assaulted and robbed once that day. Imagine Monday, crossroads and wetlands is chosen, with each side having to assault the respective sides mini-safe. This would give people a smaller target, and give people a reason to get out in the world more, instead of camping major cities 24/7.
-Items looted from the vault have a 24 hr fence time, in which the opposing faction can get the item back and stop the bonus gold going to enemy faction. In the event the item is recovered, a new vault symbol / location is chosen for the day, and the item must be robbed again.
Just some ideas thrown out.