Things appear to be working out well for MKE in this regard because of the WN/FL merger.

Today both the AirTran and Frontier operation are highly banked, and are tight for space at peak times but have low utilizaton at other times.

Southwest/AirTran uses ten gates today. Even if Southwest ultimately runs a relatively large operation at Milwaukee (like 40-50/day) they probably won't need more than 5 or 6 gates because they don't run banked hubs. And if Southwest is that large at MKE, they will be carrying a whole lot of new local traffic growth.

Frontier, who will still run a banked operation, is quite likely to get two of those freed up gates to again have D for themselves. That gives them room to add more spokes to their banks, including stacking more RJ's on the hammerhead if needed.

So for the airport:

One hub airline (WN) can handle a lot of potential additional local traffic growth if it should come, but still has gates to release.

The other hub airline (F9) will probably gain two gates, to allow them to expand if they so choose.

Two gates or so will open up for future demand / future carriers.

Milwaukee probably doesn't need any more gates for a good while now that WN is taking over FL.