Sanguinius jumped up on a pogo-stick, whine, whine, whine.
Maybe the Major Houses have Heirs Apparent and a chance of producing a Paternova, whereas Dominant Houses are dominant within the Calixis Sector giving them more immediate local power / trade / influence.
Happy Winter Holidays.
p.s. I created a number of other Navigator Houses Great & Small in my game along with various Rogue Trader lines. Here's a few that I use:
Grundenbremoch - Ecclesiarchy connected Dominant house consisting of an unusually stable Navigator bloodline wherein mutancy was never tolerated. Seemingly pious.
Laryx-Nor - Dominant Adeptus Mechanicus favoured house. Extensive use of servitors / cherubim on their vessels.
Ariomardus - Once a great power, now fallen to a status scarely higher than that of mercenaries, the Ariomardus dream of once again ascending to the forefront of Calixis politics. To this end, they finance and participate in a number of "questionable" ventures Beyond the Rim.
Tarkhunthes - A Major House that traces its history all the way back to the Great Crusade. The head of House Tarkhunthes, Hrunmer, bears a Charter signed by the Emperor himself.
Vomakt - Ancient Rogue Trader line. As long as there has been Mankind, there have been Vomakt.