There are actually multiple areas already:

::ffff:0:0/96 IPv6-mapped This space is intended for an OS or network stack to map IPv4 addresses towards higher layers, so applications could technically be IPv6 only. The packet was/will be standard IPv4 when it reaches the network wire.

::/96 and ::ffff:0:0:0/96 IPv6-compatible/IPv6-mapped These were originally intended to be used on networks to differentiate IPv4 addresses depending on the capability of the target and decide who will do the translation. These are now deprecated, but the whole ::/8 is reserved, and these addresses are promised to never be assigned to anything else.

64:ff9b::/96 IPv6-translated This is the space for IPv6 to IPv4 NAT translators. Actually the whole /48 is reserved, so you can run address multiple private translators in a single network if required. This is widely used and supported.

As a side note Teredo addresses (2001::/32) and 6to4 addresses (2002::/16) all embed the entire IPv4 address space, although they are more complex than a simple 1-to-1 mapping. They are rarely used.