The only reason you'll run out of slots is if you're building a jack-of-all trades, and that isn't optimal. It's a party based game: use the party.
As most know in the previous versions of the game, you could set each party member up with tactics and certain behaviors and not have to worry about them standing in aoe spots or have a ranged toon run up and try to melee. Gone is the ability to do that. If you want that to happen you must micromanage your game using the tac cam, which sucks for PC especially inside. The spells have always been imbalanced in DAO/DAII, but they seem to be even worse now. KE needs but 3 or 4 active spells to run the game solo on nightmare. There are other classes that can do that as well. Many spells are horribly broken and as far as I know still are. The combat and overall controls (at least for mouse and keyboard) seem to be rushed when they made it and the combat mechanics were made to be as easy as possible.
At this point, I don't think adding more slots would do any good. The overall combat mechanics, party AI, and Spells, need to be reworked. It's not going to happen for DAI. Maybe it will in the next version, but I doubt it. There is a trend of EA/Bioware games getting easier and more console oriented. Gamers now days want easy and want it on console. That is where the money is, so that is where they are going to go.