I think it is a good idea to start banking miles for a trip further out in the future especially if your main source of miles is credit cards.
You can get pretty good bonus on some of the credit cards, e.g. 20000 miles from United. Since you don't know where you want to go or what airline to use I would collect miles with either one of the 3 large US carriers. You can always use their partner airlines.
If you have a lot of Amex points there is currently a 33% promotion converting them to Delta miles. Those promotions show up once in a while.
Personally I fly enough to get the miles I need and rather use good cash back cards. With cash I can do whatever I want and just buy a ticket.