I think it's confusing to think of these as HTTP versions. HTTP is actually staying pretty much the same (with maybe minor modifications). It may be more helpful to think of HTTP/2 as HTTP-over-SPDY, and HTTP/3 as HTTP-over-QUIC, where SPDY and QUIC are TCP replacements.
Why replace TCP? Pretty much only because of the latency of opening a TLS connection. SPDY was implemented on top of TCP, with a clever handshake and support for multiplexing, etc. It seems that to get higher performance Google has decided to use UDP.
As for roaming, I don't see how the problem is changed. You still need to give your roaming client an IP address (even if you're using QUIC).
Alex