This comes down more to battle design than skillbar size tbh. The part where you say “low skill amount encourages 1 skill powerleveling” is more of a consequence than design, the consequence is whether or not you’ll gain higher power by massing one skill than searching for synergies. That is also affected by itemisation design…
Personally don’t think that the “restriction” is the problem, but rather the fact that you can “punch your way” through everything, and if you can’t do it then it will probably be a “physical immunity” which you’ll end up running from most likely unless have to kill or really good loot dropping
This is btw why I suggested the game to take a more “tekken” approach i.e. encourage all classes to get in close range for fighting unless a certain item-combination/build gives high impact from range but at the cost of other things
The problem that D2 had (IMO) was cheesy mobs (mana burn ? - 1 hit your mana is gone, lightning enchant ? - gl killing it as melee), and the fact that it was mana-potions or bust for practically every casting class tbh. The problem that D3 on the other hand had was the HUGE DEPENDENCY on lifesteal or life return per hit or life returned per resource spent sustain. Therefore IF D4 wants to be more strategic is they should introduce “consumables” on a separate bar (potions, curses, blessings, traps, summons, you name it) to “tip the scale” in your favor but without forcing you to either spam mana potions or (in the case of D3) put everything on life sustain mechanic/s
I could very well see the game being good if the bar is 4 skills, 2 mouse skills (just as in D3) but then have another bar (like on a wheel and choose up to 3) in which you choose your “strategic” choice to “fill up the gaps” in your build. Could be HealthPotions (on CD), could be HP return potions (yes, potions for lifesteal as opposed to permanent lifereturn bonuses), could be Invis, could be double armor or double damage, heck could even be massive blind (that also blinds your character maybe) or haste…, basically stuff that every class could use to “add on top” on what they have already
Which kinda brings me to the following conclusion
The more I think about “combat strategy” the more I get the impression that it’s not really in the size of the skillbar, in fact doubt much would change with it, it’s more about having consumables that matter to use at the right times
Just for reference: D2 had them in the form of antidot/thawing/oil/gas but ppl barely/rarely used these…