I think they should have gotten a bit of inspiration in the first Guild Wars. There is a limited number of ability slots, like in DAI, but a lot of abilities to choose from that make for a lot of interesting builds. It would allow a lot mroe freedom in both SP and MP.
Oh yeah, you have no idea how right you are. Yet, what worked in GW2 failed in DAI, here is why :
I've sinked 2.5k hours in the original Guild Wars and the same amount of time (for now, I'm still an active player of GW2) and trust me, the change of paradigm about skills was needed.
When each profession can have about 100 different skills to choose from including secondary professions, man that is IMPOSSIBLE to balance. And we are not talking about a bioware game, where there is always something broken which is good for the players' morale. In an MMO, balance matters and is a topic as explosive as a darkspawn's arse.
Once the original GW game passed it's 7 year anniversary mark, the PvE was full of broken bear builds/perma shadowform and other crazyness like that. The dev wanted to stop feeding this system that was almost out of control and any tweak of a single skill could have extreme consequences. In guild war's PvE, the ultimate tank is an assassin, the ultimate healer is a necromancer/ritualist instead of the monk and the best way to kill a ball of mobs was to use a warrior.
Notice the similarity with DAO where the best tank was a mage and the best ranged DPS was a warrior. That's the trademark of a system not working as intended. Sure it's gratifying for veteran players and min-maxer who have the feeling to truly master the game, but for newbs, it's insanely hard to understand.
In the end they stopped releasing expansion for GW and went for GW2 where each weapon gives you a package of 5 skills and you can choose your 5 other skills from a limited pool for a grand total of 10 skills in your slot bar.
Consequences : While some skills are much more used than others, none of them are entirely useless. The number of efficient builds did not go up, but it did not go down either and each playstyle felt very different. Same remark for weapons that all see some use. Some are more niche than others, yet it is a stupid move to go in dungeons without all the weapons that you can use.
The big problem in DAI is not the limited number of skills. The big problem is that BW has managed to create useless skills even with a limited pool of skills to begin with. Hence the impression that the number of valid playstyles are limited. It's more a problem of balance and creativity rather than number.
Here are a few examples of what I mean by "creativity" :
E.g : BW gave us lots of movement impairing spells (wall of fire, wall of ice and static cage) yet they forgot that most fights in DAI happen on vast open ground terrains that makes these tools almost useless.
E.g : functionnalities of spells in certain trees are almost the same reinforcing this feeling of blandness
- the fire tree has one CC, one AOE, one delayed AOE and a wall (which is useless).
- the ice tree has one CC, one escape tool, one delayed AOE and a wall (as useless as the one above)
- the storm tree has (surprise surprise!) : one CC, one AOE, one single target damage spell, and one AOE CC
So yeah, fire and ice are very similar trees gameplay speaking. Playing an ice or a storm mage feels roughly the same, and sticking to fire is better since it has better passives anyways.
Conclusion of my wall of text : stop yelling "there is not enough skills" because it's not the real problem. I'd like you to yell : "more variety of plastyles please" and "more skill differenciation BW please" because that's the real problem. There are fewer abilities because the RPG industry realised that it's not worth it to work on hundreds of different skills if only 10 will see the light of the day. Better work on 10 skills that feel truly unique and that get used.