Spira of FFX-2 has changed a great deal from Spira of FFX. However, many themes were carried over and Lenne played a pretty significant role in them all.
Balance
After Sin was defeated, people were delighted but confused. What was to be done from now on? Even the High Summoner Yuna decided to embark on a brand new journey for her own happiness. Where would that leave the spirit of old-time summoners? Humans seek peace at practically all intervals in history, which however is much more difficult to maintain than conflict. Meanwhile, tremendous confusion among a large population = main source of conflict in any society, Spira included.
Then Lenne enters the scene, reminding people that old memories don't always hold back development. Her presence more or less made up for Yuna's old role as the High Summoner: just as people were starting to forget the grand adventure that brought about today's peace, Lenne's song managed to bring everyone's heart back as one again. After all, we need balance between past and future to create a harmonious world.
Fantasy
Different appearances aside, Spira of FFX and FFX-2 are both idealistic worlds where the power of mind conquers the law of physics. The Fayth might be in deep slumber, but they're far from being gone. As it turned out, those who remained appreciative of the past were granted new happiness (eg. Tidus/Yuna reunion) and those who didn't had to pay a heavy price (eg. Return of the aeons). As Shuyin and Lenne's story taught us: nothing is completely black or white in this world.
I've come to realize recently that as long as I make the effort to be more honest with myself, I can enjoy FFX and FFX-2 at equal levels. I once considered most ideas present in FFX and FFX-2 "cheesy", maybe I still do, but what else is a fantasy for? We brand many of our ideals in life "silly", not because they're actually like what we say, but because we want to protect ourselves from potential disappointment: awesome ideas don't always work. As Yuna's character gradually broke away from the safety zone -- she wanted everyone to be happy without having to sacrifice anything in return, I was irritated by her naivety but also impressed: at least she's brave enough to attempt the impossible.
Endnote
FFX was a game full of bittersweet moments, a journey I would never forget. FFX-2 was a crazy sky cruise full of dangerous fun, a thrilling experiment. Frankly speaking, I can't decide which is the better game at the moment: they're really one story with one heart.