Basically in the title, I'm interested in hearing what cartridges are out there that people really wish had made it to mainstream but either died off or are still stuck in semi-wildcat limbo
Personally I really like the 280 Ackley Improved.
Normal full length action size, non belted 30-06 based case, 7mm sectional density and ballistic performance, massive almost 30 cal level of bullet choice and roughly factory load 7mm Rem Mag performance with far less powder used and with out the magnum drawbacks. Whats not to love.
If brass was reliably available from multiple vendors I think I'd definitely consider re-barreling my 30-06 to it when I eventually wear that barrel out.
I have some small hope after seeing Kimber begin to chamber it from the factory in their rifles as well as now Nosler offering reloading brass and having it officially SAAMI certified.
Who knows maybe it will get picked up by some others too