Chase is the affinity card issuer for Amtrak, every airline, and every hotel chain which accepts transfers from this program. They haven't really done anything except repackage their existing card-issuing relationships, but there are curious omissions such as Southwest Airlines and United.
It seems improbable that they could make a deal with Delta, because of Delta's tight relationship with American Express and the intensifying rivalry between Amex and Chase. They might be able to work with a foreign Skyteam carrier, but there is no obvious opening, and the carrier would know that the card would generate a high level of partner redemptions.
Diners Club Mastercard
Good point, re: Delta. I also think its unlikely to happen, but would be darn nice.
Still, if they are able to keep UA once the merger is 100% done, and BA, it would remain a worthwhile alternative to Amex for a lower fee.