Honestly, I think there are too many Quillboars in general, which makes it easy to collect a fully synergized group. On top of the changes already suggested in this thread, I'd retire a couple of the others. I'd remove Bonkers completely; they provide so much early/mid bloodgem generation, combined with decent stats. If not retired, I'd consider removing windfury from them so that you're halving their gem production. I'd also remove the 3/3 that moves blood gems to a target; blood gem decisions should be permanent. Right now there is little reason not to play all your blood gems early since you can move them to better targets later with the 3/3.
Well since not much thought was done to balancing when creating the class it's hard to know where to start. A lot of tribes need a specific set of things to happen to win, the boars have sooo many options, there is no one specific card you absolutely need, many lines can be game winning. They have way too many good cards at all tiers. In other classes some cards would be more random, or have overkill, or whatever to balance things, but not the boars.
At the core it's just too easy to get blood gems, which obviously is their fuel.
It might be easier to just buff the other classes, since a lot of cards would have to be changed in the boars.
The problem with quilboars is that they scale from early game to late game. There is no downside for playing them at any stage of the game. Just look at other tribes:
Beasts : No early game scaling, no mid-game scaling Dragons: no early/ mid game scaling Murlocs: No early/mid game scaling Mechs: No early game / late game scaling Pirates: No early game scaling, too hard to scale late game, unless exodia Demons: No late game scaling Elementals: The only other tribe that can scale through the whole game
I agree that what you should do is to remove their ability to scale mid game. I suggest to remove Bonkers completely, also move 2/5 guy to lvl 4 instead. They should be fine after that.
Not that Blizzard will ever care about my opinion (since they purposely launch overpowered stuff and shoves it down our throats by forcing Quills in every game), but here's what i'd do to balance them out:
1. Bristleback Brute - move to tier 4, change bonus to +2/+2 OR move to tier 5 and leave it at +3/+3
2. Tough tusk - move to tier 3 (it's very similar to the reborn dragon basically)
3. Bonker - change stats to 3/5
4. Dynamic duo - move to tier 5 and change stats to 3/8
5. Bristleback knight - move to tier 6, increase stats to 6/10.
6. Aggem Thorncurse - either it becomes a 3/3 or the tribe tag gets removed.
7. Captain Flat Tusk - moved to tier 5 and increase the needed gold to 4.
The quillboar tribe is the first one when we can clearly see the limitation of a 7-minion board. Agamaggam can't really stay on the board for more than a turn or you lose a minion slot. Charlga doesn't offer a strong body but it scales the rest. This means you are left with 5 usable slots. But in order to generate more blood gems you need to have 1-2 open slots to buy minions and use some battlecries etc. This introduces a lot of variance, which translates into luck and high-rolling (like there's not enough of that already, coupled with the stupid amounts of damage we take from early turns, making the game unfun for at least the bottom 4 people).
That said, it would be lovely to have a separate board slot, for a temporary minion that becomes dormant for 1-2 turns after its effect triggers (for end of turn or in-combat effects), then triggers it again and becomes dormant, etc.
It's tough, because there's essentially no useless quillboars. Short Term:
Charlga at very least needs to not buff itself. I know Kalecgos does, but Lightfang doesn't and because it doesn't scale it there's a real cost to keeping it for too long. Holding Charlie for more buffs vs selling it to get room for a better unit should be a difficult choice.
Swapping Thorncaller and Tough Tusk help the early game some (maybe tweaking stats some) so that it's a bit harder to get great Blood Gem targets.
Also, one thing Quillboars do better than other tribes is splash menagerie units, so maybe if non-golden Necrolyte only could target quillboars it'd be nice. Being able to shift a tonne of blood gems to a Cleave is huge, and if it required three Necrolytes to do that, it both becomes more rare, but also can bring a massive sense of accomplishment when someone pulls it off.
Long run, while I'm not normally on the "buff not nerfs" train, but maybe it'd be best to add more scaling to early/mid builds from other tribes.