Interesting idea
What If A Pawn Could Promote To A King?

Idea of promoting a pawn to a king may seem intriguing, but it would likely disrupt the balance and integrity of the game.

1st. If A Pawn Can Promote Into A King, Then Your Opponent Will Be Very Confused Who To Checkmate Or Not.
Why should he be confused?
He can even promote one of his own pawns to a King of his opposite (i.e. your) color.
Then he would checkmate the three Kings at once.
In this example
the playing goes like this:
1. Kc2 threatens Bd4#
1... h1=K now, after Bd4 only one BK is mate, the other one is stalemate
but 2. a8=BK and now black has only one move:
2... Kb8 and white wins:
3. h7 Ka8
4. h8=Q### (triple checkmate!).

If you kept a queen as a promotion option I doubt it would get underpromoted to more often than a rook or knight.
So I don't think it would make much difference.
As a correction though, King+nonroyal King vs King can result in checkmate.

I think it would be dumb because you win by checkmate and checkmate is when there's no moves that escape a check and your in check lets say they skewer your two king fork your too kings double check both kings then that's checkmate you also just added a peice to the board that cant checkmate the king.
1st. If A Pawn Can Promote Into A King, Then Your Opponent Will Be Very Confused Who To Checkmate Or Not.
2nd. If A Pawn Can Promote Into A King, Then The Promoted King Will Next To The King.
3rd. If A Pawn Can Promote Into A King, Then In Endgames, A King VS King Endgame Is Still A Draw.
4th. If A Pawn Can Promote Into A King, Then The Promoted King Will Still Get Captured.
Lastly. If A Pawn Can Promote Into A King, Then The Promoted King Will Not Get Checkmated.
On The Flip Side, You Are Not Allowed To Castle With The Promoted King.
And On The Bright Side, The Promoted King Can Block Checks!