Your dancing swords and baron rivendare show images of the nerubian egg.
Anyway, I'm excited too. I think more decks will integrate silences to counter these units. People will definitely experiment with deathrattle decks, and I feel like priests might present a good counter with mass dispel.
Shade is good. It takes 2 turn starts to get positive value vs. a 3/3 though, so I'm not sure how popular it'll be. Since it needs ramp up and costs 3 mana, it is could be good for zoo where you have early control anyway, so maybe it'll be viable for that. It could get value late in game as well.
Undertaker seems like a really great card. I can just imagine deathtaker -> loot hoarders -> harvest golems. It is a pretty nice ramp card for only 1 mana, and the 2 hp prevents it from being countered too easily.
Nerubian egg is very strong. It is pretty high value to silence. It would be an awesome steal for a cabal shadow priest though. But the most annoying part is that it serves as AoE prevention. No one wants to AoE and be left with a 4/4. Hell, you can even kill your own egg with a spell, or better yet, combo it with sergeant or dark iron dwarf and send it at a 3/2 or something.
Dancing swords is a pretty interesting card. While the 4/4 seems appealing, the deathrattle is really strong for the opponent. The issue is that the opponent will probably have to invest a card killing it anyway. So it is very good if you're trying to get board control. I think it will be a very popular zoo card.
Baron Rivendare will be pretty crazy. It is unclear how it works exactly, but paired with a nerubian egg kill it can be very strong. The drawback is that it is 1 attack and 4 mana, so it can get kodo'd or coin shadow priested. So I would probably try to use it when I have a deathrattle that I can trigger on the same turn.
Class-Specific
Poison seeds is neat, but I think it is a little out of place with druids considering their current mid-range playstyle. It is best when you have very low mana cards and the opponent has very high cards. So I think it is pretty situational. Maybe it could be used as a naturalize against large opponents without giving them cards (e.g. if the opponent plays a legendary on an empty board), maybe useful against hand lock. I can also see it working with a deathrattle deck.
As for avenge, it seems like it could very well be one of the pally's strongest secrets. Imagine turning a silver hand recruit from 1/1 into 4/3, which is game changing in early game. There isn't a good way to counter it except going for face.
But overall, I like the concepts. It should be a fun season, and I'm pretty sure we'll be seeing some new decks getting to legendary. I personally will be excited to play priest, just to ramp up on silences and try to counter deathrattle decks. ;D