Sadler is a great actor, but he's not in Sheen's league. That said, I don't know what else he could've brought to the character other than name recognition.
If there is one person I think would have been better than Sadler, it's Malcolm McDowell. I think a subtle Alex-esque sociopathic element would have slid nicely over Sloane's façade.
Plus, there would be that added layer of paternal intrigue. But since it would exist solely beyond the fourth wall, it would be without the contrived, cliché silliness that usually comes a long for the ride.
That's an interesting proposition. Whilst I loved Sadler's performance as Sloan, Malcolm McDowell would have taken it to another level. His performance as Soran in 'Generations' was flawless. Seeing McDowell and Siddig bouncing off one another would have been mesmerising.
The imagination runs wild....