I just hope we get some actual entropy spells.
There might be some, but maybe not an actual entropy tree. Maybe there are debuffs in the Necromancer spec, or maybe there are some debuffs in other trees (winter, fire, etc). Just because the spells are fire-based does not mean they are all nuking spells. Keep in mind for example the frost glyph, it did no damage, it was the glyph of paralysis from previous games! 
Also, entropy/debuffing spells are really not impressive to show, so I would never show those spells in demos. You might have a chance to seeing them 
Yeah... we could definitely come up with more trees we need! You cannot show us wonderful trees such as the Force mage tree or the Keeper tree and then take them away 
Yeah, I think this is due to the gameplay of the games being so so different.
In DA2, you had so many enemies and waves, that a Force Mage was not so broken as it would be in DAO or DAI. I guess that is why force mage has not come back, because they would either have to change them a lot (and people would complain), or they would break the game (cast 1 spell, win combat automatically).
Heh, I'm kinda dissatisfied with the trees generally in DA3. There are too few spread too awkwardly. Non-mages have a dump tree in the weapon they don't choose, meaning they basically have 3 trees and a spec. You can max 2 trees and a spec if they let you get to level 30, so the number of viable builds is pretty limited. A few cookie-cutter setups aren't necessarily bad, but it does hurt when your options are already limited to Mage-Warrior-Rogue. This might be intentional, but it does feel limited for a modern CRPG.
100% agree with you.
The moment I say the demo of Iron Bull having 5 skill trees, I pointed this out in the feedback forum asking for a chance to customize more our characters, maybe giving us the option to choose the spec of our companions or something. Having Iron Bull with the selection of a weapon, and then having 2 basic skill trees + his set-spec, means that all Iron Bulls in the planet will be the same...
At least we have so many fkin companions in this game, 3 of each class, that can all choose the weapon of their choice, that we have the replayability guaranteed! 