At least in the short-term no one is really going to be expanding at DTW. Just because there is more gate space doesn't mean the carriers will use it unless they feel they have the demand.
Spirit is the only carrier really increasing service. Everyone else is still trying to return to pre-9/11 service levels. DTW is still very heavy into NW, look at nearly the similar situation up at MSP. I mean, no one is going to start a second-hub operation at DTW, seriously. If anything more frequencies to their hub cities. Everyone is speculation more of the so-called "low fare" carriers will show up. Likely if they keep expanding at the rate they are, but also keep in mind they are going to put the resources where they can best make money. WN isn't going to make a hub at DTW, we all know that, so obviously they won't have trans-cons from DTW. The biggest market for the low-fares is points South, where Spirit & Champion are focussing on. AirTran could come in, but who knows what they are up to, considering the just pulled out of TOL, but they do serve FNT. CO won't increase service in DTW with the NW alliance.
Keep in mind, notice how the low-fare carriers never really compete against each other?? You can see that in action out at DTW.
This isn't really a growth year in the airline industry, its a rebuilding year.