Redeveloping Old Trafford seems to be the most desired option and I get it, its iconic and has that aura about it, but I can't help but feel the option could be a bit of a hinderance further down the track. The design and placement of OT would make it difficult to modernise & expand (especially the Sir Bobby Charlton Stand) Even if were to increase the capacity we may end up in the same position 20 years further down the road when we look to redevelop again.
I look at what Liverpool are doing with Anfield and can't help but feel they missed a trick and should have just built a new stadium.
You can also look at what Marseille did, essentially putting a new shiny roof over the existing structure. (aesthetically pleasing but not a true redevelopment or modernisation)
Bayern's Allianz Arena is a good reference point. It has a comparable capacity to OT but is modern and the fans are still close to the pitch unlike a lot of modern stadiums. The modern design makes it easier to make changes/expand from an engineering point of view. Another more recent example would be Spurs, I feel they have the best stadium in the country.
Ultimately the guts of OT also need modernising and not just the stands, roof etc. If they do decide to redevelop i'd love to by a fly on the wall to see some of the new proposals as they are pitched.