imho best way to deal with 1 ability spam is to introduce concept of "burn". Basically the more you use one skill the more it cost at each cast.
It's still the same mentality that you need to punish players for not doing what you, the dev, want them to do. Same as with cooldowns.
That's not great design, as players will feel they're constantly at war with the system. Better design means giving incentives not to spam one ability.
But really, who cares if some people like spamming one ability? As long as it's not the most optimal way to play, and there are incentives to diversify, it's fine to let players have simpler options. Reminds me of D&D design goals: the Fighter base class is meant to be simpler to play, less complex, more spammy, while other classes are meant to be more involved like the Wizard. But even Fighters can be built with more complexity, if you build into combat maneuvers for example. Maybe all classes can have simpler and more involved styles of gameplay.
I have some strong concerns for the combo focus of gameplay. Inevitably they will balance around using combos all the time, and it reduces gameplay into Simon Says. Somewhat fine for endgame boss encounters and movement, but distilling all gameplay into "do what the dev wants you to do or else, perform for us" really sounds awful.
Yup, that's the opposite issue to the one where spamming one ability is optimal. Hopefully the combos aren't forcing a way to play, and maybe the optimal way to design that would be to have multiple ways to combo with each skill, so it doesn't end with everyone having to use the same combos, or having to do the same sequence of abilities for optimal play. But like
Perkel says, that requires even more design work.
Let's hope they're aware of this. But if not, people will complain, and GGG being GGG will listen.
In fact, maybe the solution is not to have skill X combo with skill Y, but skill X being able to combo with any skill having the tag/effect Y. Taking the Druid gameplay for example, maybe the Volcano ability can combo not just with the stomping ability, but any skill that hits the ground. In fact there was also the Rampage skill that combo'd with it, so GGG might already be doing this.