The spin-off of Game of Thrones, House of Dragon is as politically charged and bloody as its predecessor. The battle for the Iron Throne continues - however, the newest iteration sheds light on the events that took place 200 years before the events featured in Game of Thrones.
Houses like Tully, Baratheon, Lannisters, Freys, Arryns, and Tarlys played major roles in Game of Thrones. However, the prequel shows little or none of these houses. This indicates a shift in power in the 200 years between the events of House of the Dragon and GOT, meaning House of the Dragon consists of great Houses that alter the power narrative greatly but are not consequential to the events of Game of Thrones.
6 major families in House of the Dragon became extinct before the events of Game of Thrones
1) House Velaryon
House of Velaryon is an important facet of the prequel. However, it is barely mentioned in Game of Thrones. In House of the Dragon, Lord Corlys Velaryon was married to Queen Rhaenys Targaryen, daughter of King Jaehaerys Targaryen I. Their children, Laena and Laenor were then married to King Visery's brother Daemon, and daughter Rhaenyra. Nonetheless, with the deaths of all major Velaryon heirs, the stature of House Velaryon subsequently declines.
2) House Hightower
House Hightower, one of the key players in the Targaryen Civil War, unfortunately, met its decline in the 200 year-span. The House is one of the most ancient ones in Westeros.
This family of highborns was marked as Kings before the Andal invasion. Otto Hightower's grandson, King Aegon II was locked in a fierce battle with his half-sister, Rhaenyra, for the crown. As Aegon's brother Aemond killed Luke, Rhaenyra's son, the war officially began.
After the battle is over, House Hightower starts dwindling into insignificance during the events of the eternal Winter.
3) House Strong
Once holding prominence in Westeros with the Targaryen rulers, House Strong soon becomes extinct. The characters belonging to the old noble House in House of the Dragon are Lord Lyonel Strong - the master of laws to King Viserys, his son and the captain of the gold cloaks, Harwin Strong, and brother, Lord Larys Strong.
Lord Strong and Harwin were killed in a fire in Harrenhal. Larys Strong died during the Dance of the Dragons, thus ending the bloodline of the noble House.
4) House Dustin
While the Starks are featured as an important northern house in Game of Thrones, the Dustins take over in the prequel. Lord Roderick Dustin serves as an important commander in Rhaenyra's army, leading her to victory many times. One such notable battle is the Battle by the Lakeshore. Other than her fleet of dragons, Lord Dustin's efforts help her immensely in fighting this war.
However, with the death of Lord Roderick, the Dustin family becomes sworn to the Stark family, losing their significance by the time the King of the North is born.
5) House Blackwood
A significant family during the events of the Civil War is House Blackwood. Lord Samwell Blackwood once fought for the hand of Rhaenyra. He later joins the "blacks" or the army formed by Rhaenyra to fight against the Hightowers. Samwell's sister, Alysanne Blackwood rises to prominence later on in the series as she pledges to marry Cregan Stark if he spares the life of Lord Corlys.
The House from the Riverlands is shown to serve House Tully in Game of Thrones, but doesn't play an important role beyond that.
6) House Peake
House Peake is a major supporter of the Hightowers later on in the series. Lord Unwin Peake serves as a commander in King Aegon II's battles. But even after the Dance of Dragons, he continues to be relevant. Following Corlys Velaryon's death, he also serves as a Hand to King Aegon Targaryen III.
House Peake is alive during the events of Daenerys's takeover, but was omitted from Game of Thrones during its adaptation to screen.
These 6 Houses may not have held prominence in Game of Thrones, but they surely hold considerable power to sway the Dance of Dragons either way.