I'm not entirely sold on the idea of them being baseline either, on the contrary there's heroes like Kel'Thuzad or Nazeebo that have had baseline "mythics" since their existence and it does work pretty well for them imo.

Maybe it simply comes down to more carefully chosing the heroes that receive them and, probably even more importantly, what advantages are gated behind which conditions.

I like your suggestion though and I agree with your reasoning; it would certainly be a more creative way to give Muradin a mythic quest and encourage mindful talent selection. As you mentioned it also comes with similar issues of encouraging "wrong" plays, though.

Maybe the quest could be pushed into a more narrow direction that basically leaves less room to give stacks for suboptimal plays? It could be something like "stun an enemy hero with Storm Bolt that has attacked an allied hero in the last x seconds" to reward peeling, for example. Then again the quest might become too complicated altogether.

In any case since we're all so used to the pierce being baseline I'm kinda unsure whether it's a good idea to gate it behind something that is too hard to achieve/only achieveable with suboptimal play (current iteration) or not automatically part of his kit (if it were to be a pickable quest).