Now, here is a thought: what if they stuck with the original plan and Crosby was cast as Troi, and Sirtis was cast as Yar? Would Crosby still have left? Would Sirtis been more interesting?
I get the feeling if the character had remained Macha Hernandez then she would still have been onboard (kinda hard to picture Marina in the role, given how Troi was the polar opposite). I would have thought Denise would have been more fed up in the Troi role and opted to leave as she did. Though I wonder who would've taken the third seat?
I could easily see Crosby/Troi bailing sooner, with Sirtis/Yar hanging around. I think the original intention was to have Data in the third chair, so perhaps they'd have moved him? Then Worf could take Data's seat and dear Wesley would still have a place to go.
I'll agree with those who speculated dividing the tactical/security duties would have been logical. I could see Worf being Yar's chief security officer, and the two switching off tactical vs. security depending on the situation.
I was also thinking, what would "Best of Both Worlds" have been like? Would Shelby have been necessary, or would it have been Yar filling in her role, causing tension between herself and Riker?