Here's the thing about it though.
Crosby is never getting traded from Pittsburgh unless he goes Pens management and tells them he wants to be traded. Otherwise he is absolutely unequivocally a lifer with Pittsburgh.
However, in the event that he does go to Pens management and suggests he would be open to a trade... At this point I would also say its fair to say that its pretty likely Colorado is the only team he would want to go play for(Assuming Mackinnon is still with Colorado at least). This would be one of those extremely rare examples in sports(At least in hockey, maybe not so rare in the NBA) where two superstar players really are close enough friends that playing together and trying to win a cup together really would be something they want to do and want to make happen.
But here's the thing about it. The Avs aren't going to pay a ridiculous amount of assets to get Sid if its clear the only place Sid wants to be traded to is Colorado. Nevermind the fact that Sid is approaching 35 years old already and while he's still a great player, he's not quite the caliber of player he was 5 years ago, and the decline is only likely to continue. So I dont think the Avs would pay up the assets Pittsburgh would want/think they would get for trading Sid. That's really not Joe Sakic's way.
I dont think we would pay anymore then a 1st + Good Prospect and then some roster forward the bridge the gap in salary cap.
It's a nice idea to think about, but I dont foresee it ever happening. I think Sid will be happy to retire with the Penguins and I think the Avs would be happy to target a Top 6 Center that much better fits the age of this teams core rather then getting a guy on his last legs for that role, even one as good as Crosby.