I guess I'll try to throw some ideas around as well. Although I've been following this thread, I can't say I remember every idea that's been posted, so I hope I'm not repeating anyone. Also, I'm not too good with names so I'm not going to make an attempt to think of any. My apologies for that.
Tauren Event/Mastery Ideas
Mastery/Event: Grimtotem Betrayal - Cairne Bloodhoof has fallen in a duel against the Warchief. Magatha, Elder Crone of the Grimtotem Tribe, poisoned the Warchief's weapon causing the Bloodhoof's noble leader to fall to an inferior strike. Taking advantage of his death, the Grimtotem have taken control of the tribe.
Event: Cairne Falls In Battle - If Cairne dies, no matter how he dies, Baine Bloodhoof, his son, will take his place. This really only should be implemented if the Tauren are a significantly weaker team compared to their enemies.
Mastery: Maintaining Balance (Sunwalkers) - Druidic representatives of the Tauren feel they have overlooked a key piece in the balance of nature. In an attempt to maintain this balance, many Tauren now look towards the sun and follow the guidance of its light. The Sunwalkers have emerged as a new powerful force within the Tauren tribes. This mastery will give you access to healing units and magic-casting warrior units.
Event/Mastery: Allegiance of the Taunka/Icemist Tauren - Ancient cousins of the Tauren, the Taunka or a hardened faction of people living in the rough lands to the north. Having failed to hold their defences against the Scourge for so many years, they now seek the Horde's assistance. This mastery/event will provide you with several shamanistic units. If you choose this to be an event, it can initiate if you capture a region in Northrend. If you want it to be a mastery they can simply appear near Thunder Bluff or a port nearby.
You could also do something involving the Scourge controlling the Razorfen Downs or the Centaur to the West, possibly even Princess Theradras and her elemental subjects in Maraudon.
I think you should add the Amani to the Gurubashi race, lorewise the amani are more "alive" then the gurubashi at this point so it doesn't make sense that the gurubashi are able to fight for world dominance but the amani arn't.
One thing that I suspect impacted his decision was the fact that the Amani control such a small area, and with the condensed map of Azeroth they're so damn close to the elves.