Okay. Let's take it from the end of the series, present day Collinwood. Characters we have are:
Barnabas, Liz, Roger, David, Carolyn, Julia, Quentin, Sebastian Shaw (?), Angelique.....
Based from that, I'd probably say that Sebastian would've left. He was the least-central character to the show, especially compared to all the others. David and Carolyn would've stayed in the spolight, because as David was still not an adult, there was still plenty of instances where he could've been put in danger. He wouldn't be there everyday, but in the later years he never was. As he grew older they could've sent him away to college or even kept him in town to develop some story with him. Same with Carolyn.
Of the core cast, I think Roger or Liz would've been axed. Personally, I would kill one of them, just to have shaken things up in the house. A mysterious death of one of the core characters, and a murder mystery to solve. Was it something supernatural? Could've been! Maybe that could've led to a new baddie being introduced.
But, of course, Barnabas, Julia, Angelique, and Quentin wouldn't have been touched. Otherwise, the audience would cry fowl.