While I do not have inside information I doubt that there will be any significant new entrants into the msp market. Sun Country has most of the new Humphrey terminals gates tied up and while they are supposed to be adding 6 more gates at the new terminal that would hardly be enough for anyone to give nw any significant competition. So far, their only competition has been Sun Country who has started regulary scheduled service.
It is amazing to compare fares from before Sun Country went scheduled vs after on city pairs that Sun Country service. It is amazing at how much it costs to travel from MSP per mile than other hubs.
Personally, I would not like to see Southwest enter the market but another major carrier expand their existing service into MSP. Let's face it, you probably couldn't go to too many places direct if Southwest entered the market. However, if UA, DL, CO, AA or HP made it a mini-hub (if space were available) it would be better for the traveling public.
Recently while spotting I did notice a Midwest Express DC-9 landing and taxiing to the Humphrey terminal but I did not know whether it is a regularly scheduled service or charter.