If TCP had a protocol specific identifier for connections (a couple of 32-bit values, for example-- a client nonce and server nonce) rather than using the source/destination IP addresses multi-homed hosts and seamless transition between different networks would become native features of the protocol. A client could roam between two different IP networks and TCP connections would "survive", for example. (I'm oversimplifying nearly to the point of hyperbole, to be sure...)
(Another fun future would have been one where SCTP got widespread adoption.)
(Another fun future would have been one where SCTP got widespread adoption.)