I had no idea Billy Campbell was considered for the role until I saw this thread.
Now that I think about it, I wish Campbell was given the role. I am not, nor was I, ever a Campbell fan, but I think it might have been worthwhile. I would have loved to have seen more than one actor play the role of the Enterprise first officer over the course of the series.
Even if Campbell had been given the Riker role, he might not have lasted too many seasons. If Campbell's Riker had turned out to be too much like the swashbuckling Okona, he probably would have worn out his welcome. But at least, the writers could have made a story arc out of it -- why he left, got killed off, got promoted or whatever.
Then they could have brought in someone else. In any case, it would have made sense that there would be turnover at that position.
One of the problems that I had with Frakes's Riker was that he started off as a hot shot ambitious officer who consistently made known that he wanted a ship of his own to command. Yet he never took any opportunity offered him.
I thought Riker's refusal to move upwards seriously diminished the character. He wore out his welcome in my view. He should have left and somebody else should have been brought in to replace him sometime during the series. That would have been a logical progression for his character's story arc.