Mcdonalds and others are pretty well established down there already, even though I haven't seen many Subways (not like in The US where there's one every corner)
Now, if I had the money the franchise that's just waiting to get established in The DR is Starbucks. I can think of a couple of "yuppy" places where this would do very well, in addition to the airports, malls, etc.
You could also buy a Tim Hortons franchise and put it in Puerto Plata (or any place where a lot of Canadians visit) and you'll be striking gold. With this Canadians would be all set down in The DR. Imagine...sun, Sankies and Tim Hortons...talk about a winning combination