I don't know about Heroic but it seems like it is possible for only the tiniest proportion of guilds in this game - I believe that is very much less than intended by the designers but only a small tweaking would make it more achievable without making it easy for all like the heroic rancor is.
However T6 tank does need a massive change. Guilds who can manage heroic should easily be able to complete T6 in 2 days, if not 1 - currently it takes even these guys several days if not a week to complete T6. The problem is purely the crazy high HP, which does need to be brought down by at least a factor of 4. The difficulty and strength of enemy attacks is fine - the problem is purely that even managing to do 1 million+ damage amounts to a tiny % of total enemy health.
