Like who? There are now very few airlines in that sector who are operating in their own right and aren't operating for the majors to feed hubs. The new generation of mid-size airliners are significant investments to be risking on an as yet unproven market.
What SEN probably needs is one of the major LCC's to do a JetBlue and bring in a sub-fleet of the new jets to open up new markets (though they seem to be doing fine with 737/320 family jets)