It would've been complicated to write but awesome.Unfortunately, Lacus' image would have been less "perfect", and Fllay wouldn't have looked half as "bad" if they'd explored this subplot so that's why they didn't do it. But it would've added some nice depth to the show and given especially Lacus a chance to shine. It's a real shame this didn't happen..
Seriously, Fllay and Lacus is one subplot SEED never bothered expounded on. I think It would be endearing to the audience because I'm all for both of them learning from each other. and would practically give a better case of a Natural and a Coordinator co-existing, through understanding each other.