Shorten some dungeons. Enhance the post-game. This can be done using such things as...

Chocobo racing minigame.

Ability to roam all areas of both Cocoon and Pulse post-game while you watch/talk to the people who are coming to terms with the changes.

Gathering/crafting minigame.

Ability to switch 'lead' character mid-battle (death of lead character causes forced change of lead character).

Exploration-rewarding minigame.

More Gran Pulse areas to explore, including an eventual discovery of non-hostile Gran Pulse habitants at a far corner of the world.

Monster Battle minigame, perhaps similar to Pokémon, perhaps similar to FFXIII-2.

Traversable world map.

Town developing minigame that leads to shops opening etc.

Two more characters - I'd go with Jihl and Serah, personally.

Actually, you know what? This is what FFXIII-2 should have been, plot-wise. The expanding of civilisation on Gran Pulse and the exploration and charting of Gran Pulse. Noel could easily have been added in somehow without getting involved with timey wimey stuff.