Why you have to disable it? If you don't use IPv6, you just leave it in default settings.

Google nexus phones don't work well with IPv6. They wake (from deep-sleep) on every broadcast the router sends, so it ruins the battery life.
I managed to pinpoint the cause using an old tp-link router that doesn't support IPv6 and comparing the the logs with WireShark.

Disabling IPv6 seems to be the best solution in my case.

More details regarding the issue (Android wlan_rx_wake wakelocks) are found here:
https://code.google.com/p/android/issues/detail?id=71372
http://forum.xda-developers.com/google-nexus-5/help/getting-rid-wlanrxwake-wakelocks-t2519294

Can a 'disable ipv6' feature be added in a future firmware?

I think that is question for TP-link support. You can report it here - http://ticket.tp-link.com/index.php?/Tickets/Submit