IMHO I don't think that Midway can or ever will achieve "major" airline status. Their market is too confined; Northeast to Florida via RDU. I don't think they go any further west than Indianapolois (any correction would be appreciated). So unless they do a major deviation from their current business plan, they'll remain the niche carrier that they are.
Frontier likewise won't be becoming a major anytime soon. Granted, they are coast to coast and the potential is certainly there, but their fleet is way too small. Given that they hub in DEN against United, the fact that they have remained low key has almost certainly been a contributing factor to their success.
Of these 3 carriers, I think AirTran by far would be the top candidate for major status. They have a major network covering almost all of the US east of the Mississippi, which for an airline founded only in 1993, they have built (and maintained) a pretty respectable route system. And with these new 717's coming in, will certainly help them grow, especially when and if Boeing announces a 717-300, and if AT launches it. Barring any Management f*** ups, a hostile takeover, a major economic downturn, or another #592, I believe that AirTran can be a major by 2005.