I hate to say it...but a TCA II sequal would be cheezy, unless, as someone else has suggested they do it ala "The Thin Man."
A THOMAS CROWN 2 might have worked a few years ago, but I think they've left it too late now. The main problem is that Rene Russo turned 50 this year - can you imagine a studio pouring tens of millions into a romantic comedy with a 50-year-old female lead? I don't think they could recast the role of Catherine Banning, and what would be the point of getting a new character for Thomas Crown to fall for? For me, the chemistry between Banning and Crown and their passionate yet very odd relationship was what made THE THOMAS CROWN AFFAIR work, and I do believe the film left scope for those things to be developed further in a followup. But now it's just too late in the day.
Also, not sure that there's any public demand for THOMAS CROWN 2. TCA wasn't that big a hit.