A party "in between" the Democrats and Republicans would be plainly right-wing and see little to no electoral success. What is growing and will continue to grow is the desire for a party in the interests of the working class.
It has historically been one of the fundamental characteristics of the American political structure that unlike other Western countries you never got a Labour party. Why should it happen now, when trade unions are weaker than ever? Who should organize it?
(it seems like wishful thinking on your part..)
Unionization has ticked up for the first time in decades, simultaneously fueled by and fueling the fast food strikes--and with Karen Lewis weighing an independent bid for Mayor of Chicago and independent labor-backed candidates winning local races in Lorain, Ohio, I think long-term growth is at least in the cards.