Could it be the fiat-to-crypto major exchange pairings?
Could it be the major company, such as TESLA or Facebook or Amazon introducing their own coin?
Could it be the some major company such as Ebay or Amazon accepting crypto payments?
No to all of those. I don't exactly know what you mean by the first one, because there are tons of exchanges out there with fiat/crypto trading pairs, and that isn't a significant factor in bitcoin's price as far as I'm aware. As for the second one, corporate tokens aren't likely to make it to market. Remember Libra? Whatever happened to that? And even if Facebook had managed to introduce it, I seriously doubt it'd kick off another bull run. And on the third, companies accepting bitcoin or any other cryptocurrency isn't anything new, and people aren't clamoring to spend their bitcoin at places like Amazon. If they are, there are ways to do it already.
Could it be the single billionaire whale entering the market?
A single one? That's a possibility, but they'd have to buy a ton of bitcoin on the open market without regard to the price increasing as they made the purchase(s). If the hypothetical billionaire were smart, they'd do that without moving the market at all, and so there would be no bull run ensuing.
I don't understand what drives demand for bitcoin or what starts these crazy bull markets it finds itself in every few years, and I only wish I knew what would start the next one. I'm not even sure if anyone knows what caused the previous bull runs, because if I recall correctly, there wasn't anything besides the halvings to account for them. Your guess is as good as mine.