If you run out of slots at level 15, you're doing something wrong. Honestly, wtf. The slots only become an issue for me at like level 20 or higher. Do you avoid taking upgrades or something?
Something wrong, eh? Really? My archer uses the cloak, poisoned weapons, flask of fire and for attack long shot (as triger for combos, too), full draw (with upgrade, to take enemies out of a fight), leaping shot (to get out of close combat doing great damage in the process), explosive shot (with upgrade, so I can spray poison) and then of course my fade-roft-ability. That makes 8 already.
With my specialization being that of the Tmepest you can't even argue that I don't have enough stamina to use it all, because I have. I activate cloak, then poison, close to my enemies, then activate flask of fire and then I can shoot long shot, explosive arrow and leaping shot within the 5 second timeframe of flask of fire, then finish it with a full drawn because my stamina is still full. I could then add the finishing touch with the fade rift ability and voila, all 8 abilities, used!
Later, when I get my special-ability for the Tempest specialzation, I will have to decide whether the next situation would benefit more from it or from the fade-rift-ability as both draw on focus, but only one fits the slot limit. As you probably know you cannot change what's in the ability slots during a fight, the character menu is locked. And when I get flask of lighting I again have to decide which of my abilities to get rid of. While it is true that I can ever only use one flask the game doesn't even let me chose which fits the situation better because I ran out of the damn slots.
For my mages the problem is a bit difficult because they most certainly run out of mana after one or two strong abilities. So filling all 8 slots with active abilities seems a bit of a waste as you better just pick the strongest. But what these "strongest" are also highly depends on the current situation and as I said before, I cannot change the abilities in the quick-slots during a fight.
It is not always so obvious that a newer, stronger ability replaces an older one so I can get rid of that. Take for instance "war cry" and "unbowed". Not the latter seems to be a replacement ability, but it is not. While it bolsters your guard even more effectively as war cry, it does not attract enemies and if you only use one tank this is what you want. Enemies shall concentrate on the tank, unbowed does not make sure of this. Thus not a replacer and you most likely end up with "challenge", "war cry" and "unbowed" all in the quickslots because they all have their role depending on the fight, how much enemies there are and so on.
All of my current NPCs save for one ran out of slots. It's a fact. If you want I can show you the screenshots.