Or maybe a zero-conf payment network? 
Sounds great but that's a false statement. I would rather stick to organized and serialized.
Since Libra will be based on the blockchain, I would hope someone can integrate Libra in LN too. Also, the merchant can accept BTC and then do an atomic swap for Libra, Tether, or whatever "stablecoin" that they prefer.
I haven't read much about Libra so I can't say if it's possible. Every Bitcoin fork with SegWit support can run the Lightning Network by making a few small changes in its code. It would be better if developers focused on improving the Lightning Network rather than wasting time on supporting another altcoin (assuming it was possible in this case).