A Buzzsaw creature that destroys any creature if there is another X (1-4...I don't know) creatures in play of the same type, tapping and possibly paying mana to do so. There aren't enough ways to really beat down on the tribal mechanic yet. If nothing else, it feels very Phyrexian, and it'd be cool to have something like that pop up again.
As far as our tribal tournament, I've put together 4 decks so far that fit into Standard. Some of them, however, don't quite fit the rules you outlined, so I may need to rework them a tad.
The purest one I've got is an all-soldier deck. I may go back and put in my Enlistment Officer from Apocalypse, but I was so close to Standard that I kept him out. I may perhaps substitute in a few Nomads, but I don't have a large block of them, so perhaps not.
The next one is a Zombie/Shade deck with a very minor component of clerics. My justification for the Clerics is that they all deal directly with Zombies...I don't know if we want to allow that or not.
I've also put together a Goblin/Beast deck, but this one also has issues with extra creature types. I put in some Wirewood Elves to speed up card drawing, a Caller of the Claw that was too cool to pass up, and a Clickslither for its combo effect with Goblins.
The one deck that most blatantly defies the rules you posted is my Riptide Project deck, named thus because it utilizes Wizards, Illusions, Slivers, and a Shapeshifter. All are directly related to the Riptide project, but it's a stretch for the format we're discussing. I could do without the Shapeshifter, but without the Wizards and Illusions combined I couldn't make the Slivers work well enough to bother with.
So...what do you think, Fell? What we discussed last Thursday was mostly that our decks be more strongly themed. I think the decks I've described fit that parameter, but the rules you set posted disallow all but one of them. Expanding the rules would probably be too much of a pain, but I'd still like to field them. Then again, I'll probably go back and rework them some now that I don't have to hold to Standard.