I'm sure it's been said ten times over, but Sergei Gonchar is not getting his number retired in Washington. I've never heard any rumblings of it, it's not on any radars.
Regarding former Caps, Alan May (former Capital player and current team in between the glass correspondent) has basically said that both Peter Bondra and Olaf Kolzig will probably see their numbers raised at some point. Leonsis famously didn't want to retire anymore numbers until he got his cup. Both Bondra and Kolzig work in the Caps organization so I think we'll see them both sooner than later. Why not Sergei if those two? Well, the role those two played is a bit different. Both were faces of the franchise during the extent of the 90's. Both were outward public figures during both high marks in the teams history (98 Cup run) and lone bright spots in low marks. Sergei never really connected as much with the local community, he's not thought of in the same regard in DC as he is in Pitt. Go to a Caps game and you'll see Kolzig and Bonzai sweaters, you'd be hard pressed to find any Gonchar's.
For modern players, Backstrom and Ovechkin are obvious choices, the team has pretty much confirmed that it'll happen and they'll be retired at the same time. Other options? Here is where time will tell. I think Holtby has a decent shot. He owns essentially every single goalie record in a 46 year old team franchise and he helped us capture our first Cup. I think that it will happen a few years after Ovechkin/Backstrom.
I think if Carlson puts together 3-4 more seasons like this past one he's got a chance.
I also think that the Capitals are also wild cards in number retirement. Yvonne Labre is one of the lowest bars you could have for a number retirement and yet he's one of only 4 retired. Even he basically said he was shocked when the team retired his number. Dale Hunter is obviously not a prolific player, but he was a huge part of the team and their image for years. So the team has definitely made it clear that between Labre and Hunter, they're not opposed to putting fan favorites who don't have the best resume in the rafters. With that all said, I think you could conceivably see a future where after Ovechkin retires Tom Wilson becomes the team captain, and well... you take it from there.