Based on the patch rundown, it sounds like Riot is going to use Thunderlords as a base line, and buff other spells to it's level. Regardless of balance, Thunderlords to performing exactly as it was meant to.
I think one of the struggles came from the fact that other keystones such as Warlords and DFT, had to be balanced around abusive edge cases. (because Brand, Tryndamir, and Yasuo exist.)
But that said, I find it very narrow to think of Thunderlord's as the only option, (though it's the most offensive one.) I personally like defensive keystones as well. (Mainly Grasp of the undying.) And when I play Sona, I go for Windspeaker's over Thunderlord's.