There's always conditional effects.
The main thing is to give us, essentially, a contenting number of options, but that doesn't mean we need three and a half bars worth of abilities by which to do that. You can always instead trait an ability to function as another ability, or with a new bonus effect, when attacking from a certain position, when within another buff or debuff's window, or when following another particular skill, etc.
And then there's always specializations, or pick X of (greater) Y number of skills, that give sufficient freedom to craft your character as you'd like without overloading your bars.
As for what I feel new skills in general should aim for, I'd say the basic idea is to give more thematic instruments that allow you to make new innovations within your gameplay up to acquisition—in other words, small steps that can have potentially large impacts. Basically, you get more points of control that allow you to get a deeper feel for your own gameplay. They should augment one's options and cause the player to consider different ways to handle different (frequent) situations. They should NOT jarringly change gameplay.
Form Shift is a pretty good example of this. When we first get it, it basically feels like it at best gives us an extra 2 GCDs of GL uptime, if we should need it. Not the largest addition, for sure, but when we go back to synced content we sometimes wonder why we were ever okay with Monk gameplay prior to acquiring Form Shift. Wanderer's Minuet, on the other hand, is probably a notorious bad example, as it does not augment preexisting gameplay or even, due to its long lock-in time, feel like an additional tool. It is basically a new style of play, with jarring contrast, acquired over a single level, non-optimal for another 4 levels, and still non-optimal over much of your rotation at level cap. A more subtle failing, imo, would be something like Tornado Kick, which adds a niche advantage in the form of a QoL addition, allowing a Monk to pull off a bit more end-of-phase damage, but is a bit too weak to have any real in-combat usage, even at seemingly optimal moments (using Tornado Kick just before buff CDs fade and only after having refreshed all DoTs and having spent all oGCDs). The same can be said of Meditation's not scaling with its current GCD, or even Skill Speed, making it all the more punishing to use in combat under Greased Lightning or with high-SS gear, even though many SS plateaus provide only a half-GCD spare to perfectly reapply Demolish, Dragon Kick, or Twin Snakes, too little to Fracture more often or adjust rotation. What these both are now should basically be what they, and most ideal ability additions, should seem like — fine QoL bonuses that offer something more to do, without being too annoying in doing it, but after perfecting their usage they should also end up situationally viable in-combat options as well, as not to fall off the battle bars completely.
Battle Litany and (the revised) Warden's Paeon are a couple good examples of niche-makers that may or may not actually be that interesting in solo content, but do give a better feel for one's role in group interactions. Dragoon becomes a sort of strike leader, while Bard furthers its support options, finally through a responsive instant cast. Those are good as well, but not an ability type I'd personally like to rely on in design. It's awesome to be able to save party members from a potent bleed or be the one job who can actually deal with Berserk's pacification effectively (if Selene is mid-heal), but when too many abilities all end up being group-content-only, you end up with the disappointment of Smoke Screen and Shadewalker whenever you're in a group with no Monk and/or a tank who doesn't know how to edge his or her dps and enmity anyways.