I don't think right-mid is that big a priority if we get a decent attacking right-back.
Petrov does just fine out there. The problem with the right hand side is that Mellberg is a centre-back playing out there. I think he has done a good job & we couldn't ask for more from him. But it does mean that the attacking is done predominately down our left.
In terms of Petrov, people say he is playing out of position, but that's not really the case. In the majority of his Celtic days he was playing behind the front two when MON played with three centre-backs, but then when he switched to 4-4-2, Petrov was on the right.
The best balance for a lot of teams is a MIDFIELDER on one side & a WINGER on the other. I'd argue that David Beckham is more of a midfielder than a winger, & that Ryan Giggs is a winger. When they played with Roy Keane & Paul Scholes it was a perfect balance. Of course, the quality of the players & Beckham's crossing ability made it an exceptional midfield, but you get the point (hopefully)
If you look at the top teams, their full-backs are all very good on the ball. This is because the way the English game is, the full-backs see an awful amount of the ball. If you have good attacking full-backs, then you can have a perfectly good balance without having a 'conventional' wide player, at least on one side anyway.
The Arsenal team that went a season unbeaten, for example, had Robert Pires on the left. But he generally played 20 yards inside, much like Petrov does. Pires had Ashley Cole bombing on past him, Petrov has Mellberg... if we signed Chimbonda, say, then this would be much more effective. With NRC holding, there is pretty much cover if an attack breaks down & the right-back is out of position, and if Petrov is playing slightly inside, it gives the opposition defence more to think about.
But what do I know, it's just my opinion!