Hearthpwn

These are the necessary cards, but most of the deck is empty. Any suggestions for things that might help with the idea?

The main issue with Thaddius Decks is not drawing both Stalaag and Feugen, but the Tracking and Stitched Tracker help a lot, in that regard, not only giving you the card but also extra copies. Another thing is that it takes a while for the minions to actually die, but in Hunter, you can force the Deathrattle with crazy tempo. For example, Let's say you have Feugen and Stitched Tracker, turn 3 ST, discover Stalaag, Turn 5 Stalaag, Turn 6 Feugen and Play Dead. It's crazy tempo. 

With the addition of Seeping Oozeling which is great if you draw just 1 piece of Thaddius, I think the hunter class is well suited to the Thaddius Package. 

What do you think could help with this deck?

Here's my take on it. Thanks for the idea, i was looking for something to have fun with :)

Why not using Cubes to trigger Deathrattles and to create two more Deathrattle minions?

Oh, great idea, though i tried avoiding using any other deathrattle minions than those two fellas so the oozeling don't copy theirs which is useless without battlecry.

Can the oozeling copy the cubes deathrattle? Has anyone tried? Seems... weird.

How about Baron Rivendare to instantly have 2 Thaddius?

I know you want to have as few DRs as possible so the Oozing copies S&F's deathrattle, but I'd still include Sylvanas Windrunner, especially considering how many Cubelocks and Big Priests are around in Wild.

I've been experimenting, and the Sylvanus is actually not bad at all. I've gotten to the point where I've dropped both oozelings, and I'm running Barnes in their place. What I find is that you moreso need to Just get either Feugen or Stalaag to die, and then instantly Summon Thaddius on Turn 6 or 7, and Barnes helps alot with the first part of that, and if you're lucky, the second part too. With Oozeling, you can't gurantee THaddius until Turn 7 or 8, which is just really slow.

Speaking of really slow, N'zoth is that. I would say you don't even need him. He's honestly more of a Tech card against more controlling matchups, but even then, he might not be instantly game winning, so there's more practice needed. Against agro he's totally useless though. So IMO, only put it in if you think your build needs it. In it's place, Deathstalker also gives you unlimited minions against control, and on top of that it's a board clear, which helps against aggro.

So from the current list in the OP, I would say - 2 Ooze, - 1 N'zoth, +1 Sylvanus, + 1 Barnes + 1 Deathstalker