My favourite troll move in CK3: swearing fealty to the HRE as King Phillipe of France traps William the Conqueror as a vassal to the HRE
One of my favourite starts in CK3 is to be King Phillipe of France, who usually loses a big chunk of his lands within the first year of the game when Duke William conquers England and gains a kingdom title (equal to your title, so he goes independent).
However if you swear fealty to the HRE immediately on game start, then William becomes an HRE vassal, and becoming king is still a lower tier than Emperor so he gets stuck in the HRE – and you get to play in a mega HRE which includes France and England.
I chose to give a gift to the HRE emperor so I could get low taxes and low vassal contribution, and within a few years I was chancellor of the HRE. A few pilgrimages, befriend schemes on the electors, and a murder later I was emperor – and William was back as my vassal, cursing his luck and doing sterling work as my marshal.
If you are trying this please note that Phillipe has random personality traits, so you might want to reroll the start a few times - becoming Emperor is much harder when he's a glutinous craven!