While anything is possible, I wouldn't hold my breath.
First, DL doesn't have a lot of mailine planes to throw around right now. DL has pulled all the mainline planes it can out of the desert. The only way to increase mainline now is to increase utilization which DL is working on.
Second, ISP's yields are pretty low and the passenger base is loyal to WN.
Third, DL is concentrating most of its effort on JFK/LGA. DL has more or less surrendered ISP. DL recently pulled the 70 seaters that were flying to ISP and went back to 50 seaters.
It is true that DL brought mainline to MHT which has some similarities to ISP. However, MHT got its mainline pre 9/11, so we're in a slightly different environment now.
On the plus side, DL's new CEO has said he wants to see more mainline flying and to see some RJ routes upgraded. I'd think if DL did start ISP service, you might get the 732 first...before moving to the larger MD88.