There's always gonna be goings-on that we're not privy to, so I won't pretend this is a definitive answer, but the DS9 Companion makes it sound as though the story idea came about before the character choice, so Worf may have been their first candidate...
"We were not told, 'This is what you have to do.' We had to figure out what to do, and Rick Berman and I didn't really know." [...] Behr put his mind to the problem at hand, and a few days later found himself thinking about a line from Ron Moore's third-season script for The Die Is Cast. "A shape-shifter in that episode says something like, 'In the future, all we have to worry about is the Klingons and the Federation, and that won't be for much longer," says Behr.
"I'd said to Ron at the time, 'You know, we could do a whole show about that if we wanted to, how the Dominion would want to get between the Klingons and the Federation.' But the earth didn't move. Nothing shook."
Still, the idea stayed with Behr, so he casually tossed the notion at Berman, who had a much more spontaneous reaction. "Rick said, 'The Klingons - that's the way to go,"" recalls Behr. "Everybody loves the Klingons. And if we bring in the Klingons, why don't we bring back Worf?""
The suggestion gave Behr pause. Bring in a new character, or rather, a new old character? From another show? But he had to admit, it made a certain amount of sense. "Of all the TNG characters, probably the one who would fit in the best [on DS9] would be Worf," he says.
Berman saw even deeper possibilities for the character. "Worf went from being the only Klingon in Starfleet to being the only Klingon in a Starfleet that was no longer on a friendly basis with the Klingon Empire," Berman explains. It was a storyline ripe for development.
