First question: why is a player even trying to singlehandedly introduce something on the scale of a whole kingdom into an established setting? (I'd almost certainly hard-no this right away to head off subsequent issues noted below)
I'm for DM Authority, but different DMs invite different amounts of player input.
I personsonally go into Session 0 with hooks and ideas to get the players excited, and do a lot of world building after that. Last campaign I ran (4.5 years) didn't have a pantheon - no divine characters, just didn't come up. On the other hand, Session 0 for my most recent campaign started with the player wanting a druid to have a "real connection to the magic of the land as
something" and proposed that the moon was the skull of a decapitated god and the world itself was it's body. Hmm, does this get int he way of anything? Nope. From there from player suggestions we had the dwarves had been genocided, the drow were a created race to take their place (for the victors) mining the Bones of the Earth (literally), and the Halflings were also a created servitor race.
None of that got in the way of what I had planned, and made the setting both unique and having lots of player buy-in. So sure.
A kingdom? Not a big deal.
Actually, I realize I
have introduced a kingdom as a player. Was playing in a world with one continent fleshed out but plenty of unexplored and undetailed lands. I was playing a halfling and when the DM asked me to detail where I was from, having been a bit enamoured with the Eberron Telenta Halflings that domesticated and rode dinos, I asked if I could introduce an island that time forgot like that. He agreed, and poof - I've added a kingdom.
Which brings it back to DM Authority - in both cases the players were suggesting things, but the DM (me or mine when I was a player) had veto power. "No, that doesn't fit." And everyone would have accepted it without an eyeblink.