What's an alternate universe look like where Blackbeard found his devilfruit first, and Whitebeard's crew had no issues with him taking it, as is the rule for their boat?
The story kicks off because he betrays the crew, but in a world where he didn't do that, how do his plans change?
There's something vital about Teach's mindset: his plans aren't 100% certain. He knows he leaves things to chance, and we saw it in Impel Down when he faced Magellan. He's even surprised to be alive when Shiryu saves him.
If he had gotten the fruit, he wouldn't have hesitated to leave, and in fact, he would have had plenty of time to gather followers across the seas. From what we've seen, Whitebeard let his children form their own crews (e.g., White Bay).
Their priorities were going to be the same. Let's review some events in the story:
-Being a Shichibukai: To achieve this, he primarily wanted to deliver a pirate worth around 100 million Beli to the Navy. After Skypea, he had his eye on Luffy. If it weren't for Ace not being there, Luffy would surely have been captured by Teach on Water Seven (of course, depending on whether Garp arrived before or after him). The Marineford War would have passed. Garp didn't intervene to get Ace out, and he won't do it with Luffy, because Luffy would react the same way: he chose that path.
-Marineford: Ace obviously shares brain cells with Luffy, because he would definitely break down the main door of Impel Down to go and get him. Blackbeard and Ace would meet, but thanks to another character, Ace doesn't confront him and they leave. And well, now MARINEFORD, you have the Navy waiting for the Revolutionary Army, but not only them show... but Whitebeard's crew too.
Why? Well, because narratively it makes sense. Ace already showed everyone who his little brother is, and he immediately went to find Luffy in the damn Impel Down. Sengoku knows who Dragon is, so does everyone in the government high command, and that's why Luffy is bait.
The battle ends with Whitebeard's life anyway. The revolutionaries take Luffy out, and Ace, well, you can choose whether he lives or dies. I already made another post answering more or less how it changes if he lives.
So as you can see, many of the paths (if we put them in perspective on the goals Teach showed in the story) result in Teach becoming emperor.
So yes, no matter who found the fruit first, Teach was likely someone inevitable.