For the sake of transparency, these are the houses declared by GMs or those players have privately told me they are interested in:

[indent]▸ House Targaryen
▸ House Baratheon 
▸ House Celtigar
▸ House Greyjoy 
▸ House Hightower
▸ House Mallister (current Lords Paramount of the Trident)
▸ House Martell
▸ House Payne (current Lords Paramount of the Westerlands)
▸ House Stark 
▸ House Tyrell 
▸ House Velaryon [/indent]