Very, very little chance even a "conservative" Democrat could take the 3rd District, as it covers the most populated portions of the ultra-conservative Utah County area. I'm seeing some speculation he'll try to run for Hatch's senate seat. While I'd love to see Hatch out, I'd hate to see Chaffetz self-limiting his term in the House just to step up to the Senate. Politics seems to corrupt the vast majority of politicians within a few terms, better to get them out entirely than just shuffle them around. I'm hoping he's stepping out for good reasons, based on what he said, and not to just run for the Senate.
Edit: Just re-read and realized Hatch is up for re-election in 2018, and Chaffetz specifically said he's not running in 2018 for anything, so there we go, hopefully. I do hope someone uproots Hatch in the primaries...