Had it's flaws but one thing I liked about the "Pond era" was the extra dynamic they had when Rory and River and co were on board. I wouldn't want it to get packed, like with the 5th Doctor, or some of the 1st's, but I'd quite like an ensemble on the TARDIS, with several different companions, possibly from different times and places, with various different relationships with the Doctor, maybe have the sort of cosiness that came with U.N.I.T. etc? Could have your average, 21st century girl companion, but maybe also an alien or something who doesent get how to interact with others, or an initially untrustworthy person maybe...