Physical damage is *resisted* by enemies. You mean that some enemies in Origins and DA2 were potentially IMMUNE to certain damage types (which was a problem). Granted, there were weapons in DA2 that converted melee and ranged damage into a magical damage type, so you could also run up against that problem on a non-mage.
If the *immunity* problem isn't there (and they don't have situations where you're fighting 6 different types of enemies all of which are immune or highly resistant to a different damage type), then it shouldn't be a big deal. You set your loadout when you get to the area, run the area, get to a new area, reconnoiter, reset your loadout. Sounds like actual fun to me.
Now, if they wanted to be REALLY awesome they'd let you save several different loadouts so that switching would be just a matter of picking one from a list . . .
Physical damage was never resisted to a point in DA2 where you would need to switch away from it. Whereas elemental magic was worthless against certain foes, making either the mage as a whole or the ability trees useless.
Your solution is just aggravation: know the encounter in advance, deal with the pain of switching out abilities you already have, when you could have picked them in the first place.
The strategy in D&D in terms of spells wasn't from the limited number of spell slots but rather the limited number of uses. It was "this fight" or "some other fight" type of choices.