Since debuting in 2000, the Head of Household is forced to name a replacement nominee.
Seasons 3 and 4 allowed every Big Brother houseguest to compete for the Power of Veto, but the format changed in season 5. Though it was tweaked slightly after season 5, the Power of Veto format has remained mostly unchanged for nearly two decades. Since season 5, the format has allowed only six players to compete for the Power of Veto. In seasons 5 and 6, the Head of Household and the two nominees each selected one additional houseguest to compete in the Veto competition. That rule changed in season 7, and has remained the same ever since. The Power of Veto competition now includes the Head of Household, the two nominees, and three players chosen by random draw, with a few "Houseguest's Choice" chips added to the draw, allowing occasional HOH or nominee-chosen houseguests to fill one of the slots in the competition.
The Power of Veto is a foundational element of the Big Brother game, but it's in dire need of some tweaking. Although the six-player format has facilitated many memorable moments over the years, including Big Brother 5's game-changing "backdoor" plan, the six-player rule has seen diminishing returns in the modern era. The main problem with the limitation on Veto competitors is that the odds favor the majority, meaning that seasons already plagued by the dominance of majority alliances are likely to become even more predictable. If the format allowed every player to compete for the Veto, it would give targets a better chance to save themselves, rather than being "backdoored" after never having gotten a chance to compete for the Veto at all. It may seem regressive for the show to return to the original rules it moved away from twenty years ago, but it's a change that could lead to more a dynamic game in the current era.
Another change that could improve the Power of Veto's effect on the game would be for the show to adopt in the American Big Brother version.
The biggest issue plaguing the modern Big Brother game is the prevalence of big alliances steamrolling the game with little resistance, leading to dull and predictable outcomes. A change in the Power of Veto format could dramatically lessen the frequency of steamroll seasons, as there would be far more opportunities for houseguests to save themselves and alter the trajectory of the game. The change has had an undeniably positive impact on Big Brother Canada, and there's no reason to think it wouldn't have similar results in the US version of Big Brother.
Big Brother 24 premieres Wednesday, July 6 on CBS.