As for now some ISPs are starting to provide ipv6 support to their clients. Usually it's done properly with DHCPv6 and Prefix Delegation, but sometimes they just route /64 to their switch, giving you the gateway. As far as you get connected directly or via L2 switch this works fine. But as soon as you want to get a router between their switch and your LAN everything stops working (ND packets can't pass through the router and we have no nat here for IPv6).
Why not implementing NDP proxy in RouterOS? It's not a good way of designing networks, but we have arp proxy for ipv4 already and it's quite useful. In my opinion NDP proxy might be very useful as well.

according to rfc4389 ND proxy requires that ports (interfaces) cannot be bridged. If bridging is possible that should be used instead. Also, RouterOS has an option to apply ip firewall on bridged traffic.

+1.