I wouldn't have a problem with any of the above as long as EVERYONE had access to it. The problems arise when only a select FEW have game changing additions.
Then the term you're looking for is something like "too advantageous," not "unbalancing." Economically they're very balancing.
me personally, I wish they would just put up a moongate-like portal that you could go through as often as you wish to go back and forth to any shard you want to (on second thought I don't want atlantic a**sholes raiding my spawns)
barring that it would be nice if you had a common bank box for each account that you could access across shards - put something in it on one shard and take it out on another
and since I know that would result in epic pancakes from people, as a last resort, I'd like the ability to shop vendors on other shards with this new vendor system - even if I had to pay extra
This would require a massive overhaul to existing item code to prevent duping and lost items. It isn't hard to code safeguards around normal maintenance schedules. As a safety buffer, I'd put it at ±60 minutes of a shard's scheduled time. But this still cannot account for any unexpected shard maintenance.
Moving an item to another shard is not as simple as moving an item on a shard, even from one container to another. It requires destroying the item's original information and recreating it: item ID, CC container and location, EC container and location (or facet and coordinates if not in a container). This is going to be a lot of server load -- a genuine higher use of CPU cycles, not just item count. There would have to be a limit per unit of time to prevent scripters from trying to break the system. I'd personally code it so that while an item should transfer a lot faster, it wouldn't be available on the destination shard for a full hour, after which time another item can be transferred. This way, any unplanned server maintenance would cause the transfer to abort, and the item would be recreated on the original shard. Always, always assume the worst intentions with any given user, that 1% who are looking for a "But I followed your rules!" excuse. Stress-testing and bug-hunting are things I've done more than a little of, and I've routinely discovered bugs that none of the formal developers conceived of, because they couldn't put themselves in the mindset of someone looking to game or outrightly circumvent a trading system.
Finally, you said you'd be willing to pay extra, but how much, and how many other players would? Transfer tokens are revenue generators, and EA would have to replace that revenue somehow. Lots of players won't want to pay more for a feature that they wouldn't want to use, or in the case of 14+ year vets, a feature they already have. How much extra would a player be willing to pay every month for enabled transfer? With an hour limit, that's 720 items per 30 days, a bit more than a $20 token, and you're talking about multiple directions, multiple shards. An additional $5 would be a tremendous value for players, and a possible revenue loss for EA, unless it somehow encourages a lot of players to sign up for it. Ten dollars would be more realistic, but then would there be enough interest? If the Dev team could devote resources to making this happen, it would have to be a pilot program whose price may increase, or it might be scrapped without enough interest.