I think STS has done a great job with last couple bosses fights, specifically Fragx and the mystery mash boss. It made team mechanics mandatory or introduced the odd AO like boss fight quirks, (Think Gurgox and Overlord).
How about some others? What if...
A mob of undead swarm, no big seen it all before huh? But no attack can damage them, out team takes a beating trying to figure out what to, and then the Mage has to heal the team and, WHATthe HECK! The green glow appears on every enemy in range damaging (healin dead) and dropping their seemingly invulnerable shield. Victory!

We move on encountering two odd enemies, we approach, they laugh and one of them buds off creating 3. Then again one of them splits into 2! Our team can't kill them fast enough, they allmost seem to spawn just a little bit quicker than we eliminate until our tank succeeds in pulling ALL of them into a tightly packed stunned group. Finally they can be brought down!

Lastly we see another new type of foe in a wide open map. First one seems to die easy enough the tanks confidently plow ahead! Then they buff, and we see them mocking our attempts to kill. Faster then any elixired toon they dart in and out of range. Becon pulls them but they are so fast they simply walk normal speed away. They can't be hit with mele and they seem to know the range of our attack, speed kitting.away. Our archer laughs havin figured t out, "wait untill I root them, they too will fall"
The team now waits to archers to set up kills.
And we all plow thought them like a chainsaw through warm butter.
What other ideas could y'all come up with?
I know it could have gone in suggestions but I'm just brainstorming.