If the Warehouse 12 gets a go ahead, they better call it Warehouse 12. That way it leaves Warehouse 11, Warehouse 10 and so on up for grabs so we can have a new spin-off every few years^^ (love that show).
One thought I had, given that
is that they could retool the show into a sort of anthology series following the adventures of Warehouse agents from many generations throughout history. The episode "3...2...1" showed that such a broader view of Warehouse history can work, and indeed the worldbuilding of the history/mythology of the Warehouses is just about the most successful element of the series. Maybe you could have a "wheel" approach -- an episode about the modern cast, then an episode about H.G. and her team, then an episode about some other Warehouse team from another era, then repeat the cycle. So if they did, say, a 21-episode season, you'd get 7 episodes with the main cast, 7 with H.G. and her bunch, maybe 3-4 with Jack & Rebecca in the '60s, and 3-4 about various other teams and eras.