LACC, should the Farmers Field project get done, will equal SD's pending new convention center size, but that is a few years away at least.
If the area surrounding it continues to grow, attract restaurants (which it has more than I thought it would these past few years), and have more public transportation options open up (as you mentioned about the Expo line), then maybe the next time Comic-Con's contract is up, they might consider it. I realize keeping options open is important, but (as far as I know) Comic-Con has not signed a very long term contract to stay in SD. Could they themselves be seriously thinking about leaving or using its potential as a leveraging tool? I sincerely hope not, since my annual stops at the Carlsbad Danish Bakery (can't recommend this place enough) on the way down to Comic-Con and gluttonous breakfasts at Richard Walker's Pancake House would be terribly missed.