I wonder if turning the New Orleans Centre into a great big sports mall would work? Maybe put some restaurants/bars in it, some sporting goods stores, museums, etc? The ESPN Zone idea would be good too. ESPN Zone would be a draw all year long, and would likely pull a lot of tourists from the Quarter to the Dome area.
If anyone has been to Wrigley Field in Chicago, there's a ton of small bars/restaurants around the area that really make a huge contribution to the atmosphere surrounding the park. I'd love to see things like that pop up also, a lot like what the Fulton Street area is becoming. Hopefully similar places will spring up down Poydras to bridge the gap between the Riverfront and the Dome area, if there's something to draw people up that way.
I've always lamented the fact that Zephyr Field wasn't built downtown. It would have been great down there. Maybe they could put like an off-field Zephyrs watching place so baseball fans could watch the Z's in a good downtown atmosphere? I don't know of any team that's ever done anything like that before, but race tracks do it, so maybe it would work?