It took MikroTik 8+ months to develop v4 from v4beta1 to v4.0. A similar amount of time taken on v5 should get IPv6 on everything.
All routing protocols seem to be working, BGP, OSPFv3, RIPng. Firewall Filter & Mangle are also there. ND & PPPoE seem to be coming along.
Things to add or finish before 5.0 is finished:
Firewall Connection Tracking
Firewall Layer7 Protocols (or just let IPv6 rules use the IPv4 L7 definitions)
Queuing (Simple Queues,etc.)
IPv6 addresses able to entered into any address box (ntp, dns, Netwatch, watchdog etc.)
Telnet/SSH/Winbox client & servers
web interface, webbox/webfig, webproxy
MPLS, VPLS, IPSec, EoIP, PPTP, PPPoE, OVPN, L2TP, VRRP clients & Servers
IPv6 Pools for PPPoE & DHCP
DHCP (Mikrotik seems to not want to implement this)
Wireless Registration Last IP field
everything else that currently can do IPv4 (except for IPv4 only things like NAT,UPnP,etc.)