Assuming that DL and CO really do break up.

You can either get one of the Chase Continental Cards http://rewardsdb.com/CC-Airline-US.php the World Mastercard & Presidential Plus Mastercard both have no annual earning caps which would work for your spend.

Other options would be an American Express card with Membership Rewards (which can be converted to CO miles), or one of the hotel cards (although Starwood which most people usually recommend on FT for everything regardless of the request has a bad redemption rate $2 per mile for CO so maybe that isn't the best option).

The Chase Continental World Mastercard may be the best since you get 1:1 miles, discount certs, and 2 lounge passes annually.

That all said, until there is actually a definitive answer about CO's future I am not sure you would want to do this. When the dust settles you may find CO doesn't go where you want, or DL/NW is the more convenient routing etc.....