Me personally, I’d like to see them enforce some of the lore of the game. Having 2 of the same epic, legendary, or mythic in the same deck simply doesn’t make sense. “Named” cards are named for a reason - they’re unique people, beings, or creatures.
That being said, I think the real solution is to change the synergy formulas. Make the kingdom/type bonuses higher, give penalties for illogical combinations, and even consider group traits for well-synergized teams.
Almost any 4x team is bound to be annoying, but they will have a weakness and be very beatable. 4x Astral (or 3x Astral + Maw) is really irritating, but loses badly to Mab apparently.
(I think that the ratio on Astral’s ability needs to be 3:1 or 4:1, serious power creep happening there… And/or make it significantly harder to use, who knows…)
Rather than impose limitations that cut down on creativity and so on, I instead suggest ONCE AGAIN that the kingdom/type buffs be improved to a point where they’re actually worth trying to build teams for. Maybe bring percentages into the mix so that it scales into end-game numbers - that +2 attack doesn’t mean a damn thing to me but +25% might.
Of course, the tiny amounts (4-5 only) of various troop types we just got left with prevents that kind of change from being doable at this time. Something to consider doing in the near future.
Not that I ever want to see 4x anything again, I just think the ‘buff everything else’ method would serve us better in this case.
Not only have I whupped a 4x fully-traited Astral Spirit team, my L167 phone alt has also whupped the same team! Some of you guys need less tears and more whupping cream.
I think the intent is very noble Jainus, but unless some troop is clearly broken, like Dust Devil was at the time, there is no reason to go into such extremes.
If we would happen to have a restriction to only one copy of a legendary troop on each team, then it would at least make some sense.
I think this would just hide problems, rather than solve them. I think the same thing about one-shot spells. If something’s so powerful that you think it needs to be limited, then maybe it should be rebalanced, rather than just prevented from being exploited a particular way, leaving it still there to be exploited in some new, creative way that hasn’t been discovered yet.
Stacking tends to exploit local optima. If the troop isn’t overpowered, then it probably has a weakness that can be exploited to take it down easily. I really am starting to think we could use some spell damage resistance traits. That might help counter the stacking empowered damage spells that have been showing up lately.
This in an interesting discussion, especially if you also consider that there was another discussion about doppelgangers a few days ago. I would suggest to combine a bit of both ideas and do as follow :
Those two combined game mechanics would also open a new space of trops design, allowing for example to create new troops that would be powerfull when duplicated in a team but with uniqueness restriction so that a Doppelganger would be required to achieve the desired result.
But all that is hardly a necessity, and I guess the devs already evaluated those options since those ideas are far from new or really original ones. Yet, since there was a discussion here, that was an opportunity to speak about that.
Maybe time to look into this again with all the 3/4 stack teams.
Most CCGs do have this concept in them and it is a mechanism that helps maintain balance when new cards are released. It also just ‘fits’. Why would there be two of Emperor Kovash in a fight?
Unique, named cards should be singular.
Multiple instances of “orc” okay, I can buy that.
Multiple instances of Elemaugrim. No, that makes no sense.
In my opinion every form of ranked pvp should be limited to 1 copy of any mythic, and 2 of any legendary.
But in pve and casual, go crazy.
game looses income as ppl will want only one copy of mythic everything 
and the money already spent on duplicates - how would you solve that?
but i think new game modes that arent introduced yet - could be introduced with such restriction without giving the above issues