[Balanor]

Especially as it is limit: one per character... Is there a chance we can get the ability to sell property back (hopefully at full price since, well, it's real estate which is -quite- different from other things which depreciate, usually real estate is an investment and only appreciates, but if it doesn't appreciate at least it won't depreciate), in order to upgrade?

Obviously you haven't tried to sell a house in the US over the last couple years. Speaking from hard-learned -and very costly- personal experience, I can tell you houses do not always appreciate - and can absolutely depreciate.

Regardless though, there is already in-game methods to try and upgrade to a better place - contact other PCs, post a message on the mesage boards in-game advertising your place, or something similiar. Just like in real-life cases, a person does not try to sell a house back to the bank/city/state/person that they bought or acquired it from, but rather to another individual.
Or maybe start up a real estate business in Thay! I can see a slogan now - "No Agents Killed by Dissatisfied Red Wizards in almost 2 Months!"

Also, you hear alot about estates in in-game dialogues... I assume that's still on the wishlist and being worked out?

Yes, this is still something on the wishlist. This is planned to essentially be one of the last steps in the Path to Power for individuals on the Merchant/Mercenary path or for those who rise through the ranks of the Legions and retire from the service. Becoming an estate owner would be the opportunity for the individual to truly become a 'noble' of Thay.