I would take X as base and then add features from X-2:
The customization system, the Coliseum, the roleplaying, the Via Infinito.

I think X works as a linear game. I did however love the roleplaying aspect of X-2, as in your choices affected the outcomes. The Via Infinito is my absolute favorite optional dungeon in all FF and the Coliseum my favorite "sidequest". Combat System is pretty much tied for me, but I HATE stat-maxing and I think it ruins the optional challenges of X. Therefore, X-2's customization system was much more appealing and it also makes for a very versatile system.