Dunno if some person suggest it either but.

The possibility of moving a house would be great.

By that i mean for exemple, you have a S one and want a M one.
Funitures goes in the npc that keep object like for auto demolition. Object that can't be move like walls get in the new randomly (if you don't lose it you can move it where you want a bit, that's my point as some items are fairly expensive).
For fc house, things like ship and rooms be transfered as is.
And paying the whole price of the house I suppose.

The problem actualy is that if a person want to move houses, well it's impossible without the possibility to lose everything.