Some of these needs/wants seem to be contradictory...
The primary problem I see is that you want to do a monthly subscription but don't want to provide ads... if you are not showing ads then what does paying for the premium monthy membership actually do for the player? It looks like only some premium only areas, but since you want everyone on a level playing field you have to make sure that you can't become more powerful in a premium area then a non-premium area so why bother going to the premium area other than for the flavor of the zone.
Premium only items only avoids pay-to-win if the difference is cosmetic only... if you add premium only areas and they contain premium only items I can only see that leading to a time where there becomes a point that you are forced to become a premium member to have access to the highest level stuff because the premium members will want something beyond an alternate start area and would prefer an alternate end area.
You say you don't want Premium vanity items... but I think this is one of the few safe areas to give premium bonuses. To avoid pay-to-win you can really only give players the option to pay to be cooler... which pretty much translates to vanity items.
Require premium for respec seems like a bad idea. The best alternative I can think of is to limit how often players can respec and allow premium members to do it more often.
Extra stash is good, when premium ends I would say that the extra stash becomes withdraw only.
Here are some Ideas I thought of while reading your post...
If you have a crafting system you could make it such that a premium membership is required to craft the high level items/consumables but any player are able to buy them. If having housing/shelter is an important aspect of the game you can make it such that only premium members are able to own/build housing and free-players have to "rent" housing using in game currency. When a premium player quits being premium, the players that are "renting" still pay rent, but the money disappears, and the house goes on sale for purchase by other premium members.
I don't see any reason to disallow free players from using in game currency to buy premium only currency from premium players. This will soften the wall between pay and free users and will give pay users a method for quick cash infusions, you might actually get more sales of premium only currency b/c of this than you would otherwise (Look at how kingdom of loathing handles Mr. Accessories for a good example).
So basically, you could try to make it such that all players free/premium are on equal footing for the action/adventure portion of the game... but it's the premium players that are fueling the economic bits.
Maybe have an in game store that sells items like
Commercial Licence: cost 1 pCoin, upon use allows player to list items for sale for a week (player-to-player cash in hand would still be viable for free players, but an interface that keeps your items for sale while your offline wouldn't be)
Manufacturers licence: cost 1pCoin, upon use allows player to craft high level items
<other type of license>: cost 1pCoin, allows player to cultivate some premium only resource... maybe a high level metal for blacksmithing or high level plants for cooking herblism
Exp Potion: Cost 3pCoin increase xp for timeperiod
Vault: 5pCoin: increase stash size
The store sells licenses so that a premium player can spend x pcoins to gain y licences to sell for z gold.... now the free players are not entirely blocked from the premium actions, but the premium players don't mind because they are getting something out of the deal... rich.