I would like to have seen Bret return to the WWE, and I do think he would have come back.
Think about it, Shawn Michaels was not around at the time of WCW's demise, he only came back in 2002, so Bret would not have had to worry about dealing with Shawn on his return to the company. If HBK had been there, it could have been a major barrier in Hart coming back to the WWE.
In 2001 when WCW went out of business, Bret was still a really big name in the industry, so I think its likely that Vince would have been interested in bringing him back, even if it was simply for the financial/merchandise benefit on having Bret back in his good books. We know how desperate Vince was to release the Hitman DVD, as Bret is one of the most popular wrestlers of all time, and has a goldmine in his back catalogue of matches. And if he was still healthy, I think the pair of them could have put the past behind them and we would have seen Hart back in the WWE.
However, there are a couple of reasons why this may not have happened. Owen Harts death in a WWE ring and Bret's anger at the stupid stunt that went wrong could have been a big stumbling block to his return to the WWE. In 2001 the tragedy had only happened a few years previously and who knows if the scars had healed, or whether Bret would have been ready or willing to return to a WWE ring.
Secondly, after the end of WCW Vince didnt really bring in many of the top stars. Yes Booker T and DDP arrived almost immediately, but we didnt see Goldberg, Steiner, Nash, Hall and Hogan into over a year later. Also, there were plenty of stars such as Sting, Lex Luger, Piper, and Sid Vicious who never made the jump over after the buyout. Maybe Vince would have ignored Bret like he ignored these guys?
Thirdly, Bret was on a ridiculously high salary in WCW, would Vince have been willing to match that? He originally left the WWE after Vince deciding that he was no longer worth the money they were paying him, so why would he want to pay double that to have Bret back? Also, would Bret's pride allow him to accept a greatly reduced salary?
If I had to make a bet however, I would put money on Bret returning to the WWE, maybe not immediately, but I think he would eventually. With opportunities to wrestle guys like Kurt Angle, Chris Benoit, The Rock, Chris Jericho and Undertaker, there would have been so many great matches we never saw, or never got to see again.