so now after the release of the new expansion Forged in the Barrens it's worth looking a bit closer at card interactions and natural synergies emerging from a simple game mechanic: gaining one mana crystal each turn.
There are cards that are quite versatile and are only archetype bound. Twilight Drake and Doomsayer are good examples.
While Twilight Drake is useless in Aggro since it's usually out of cards at turn 6 and Control as it doesn't provide any Hero protection or immediate board impact on turn 4 it specifically shines in any Combo deck that wants to draw cards early at turn 3 with 3 Mana cards like Arcane Intellect, Sense Demons or Thoughtsteal. Doomsayer is great in any Control deck that wants to stall and reach later turns for more powerful cards.
Twilight Drake and Doomsayer can both be played in almost any class as they are not really class-bound or specific.
I wanted to discuss this in a forum thread since I believe we can find hidden, vey efficient combos much easier as a community. Such combos often times come up in the same expansion but also just emerge after multiple expansions. These of course require your first minion not to be destroyed before enabling that mini-combo. Here are some examples of Neutral-to-Class card synergy:
While you may not run the latter two in an elemental deck, you will likely still have some spell discovery that can generate those cards. In theory pretty much every class can make use of a tribal synergy, but Shaman effectively has three of them between Murlocs, Totems, and Elementals, so they tend to benefit most from this card imo.
Frankly, I think there's a reason this line of exploration is hardly explored. I'm pretty sure it's better just to have a good three drop and a good four drop than a mediocre three drop that likes to be followed up by a specific mediocre four drop.
In later expansions this deck was buffed by Imp Gang Boss (in combination with Blood Imp), Imp-losion (which also benefits heavily from Spell Damage) and Mal'Ganis. This Swarmy theme is in every class btw. (Druid has Treants for example).
Both deck types are Combo because they exploit the weaknesses of a Control Deck, board clears and Hard removals. Vs. a "Big" deck a Control deck wil simply run out of hard removals, and a swarmy deck that can fill the board with tokens over and over can overcome any amount of boardclears. After a while Deathrattle decks came into play as well to beat Control.
Funnily enough, if you try to make an aggro deck with Warlock and look at the reward cards, there is also Grimscale Oracle for a level 53 and 54 Warlock Class. Guess what the Aggro deck looks like? :D It's a murloc deck. Suddenly a Felstalker works great with so many 1-drops. Also not all Discard cards are played in the same deck, I'm trying out Soulfire in the Spell Damage/Swarmy deck and it just kills. A 6-damage Soulfire with that amount of Spell Damage (along with Bloodmage Thalnos) isn't that rare.
The real challenge is to find the appropriate Neutral cards to fill in the missing spots to 1. still have a great curve and 2. have enough synergy to be able to compete. With the Swarmy Deck I'm playing around in Standard right now, I have won some games vs. Quest Decks already, still refining though as I'm sure I'm missing the last bit. Rustsworn Initiate though works great with it already (it summons a Spell Damage Imp which helps with board buffing and Grimoire of Sacrifice. Also it's another minion from the "Rustsworns" (like Rustsworn Cultist) so I guess that's no coincidence.
What's worth mentioning here is that usually Combo minions have even stats (or close to) much like Ogre Magi or Kobold Geomancer as they are neither aggressive nor passive, but rather situational in combination with Spells. You see that with most Combo minions which sacrifice stats for a better ability.