Favourites and Abilities issue

I would argue that the cooldown reset for all powers after switching is there for a reason. With the individual power cooldown system (which IMO is superior to ME2 and ME3's global power cooldowns), Bioware wanted to strike a balance so as to not return to ME1's power spam at the beginning of every battle, since ME1 had like 7 powers for Shepard, all on individual cooldowns.

The cooldown reset is there so that you can't just keep switching favorites to spam powers infinitely in a short amount of time. For example, if you could, then you would just use 1. Incinerate, Overload, Pull -> Switch Favorite -> 2. Throw, Singularity, Shockwave -> Switch Favorite -> 3. Annihilation, Charge, Cryo Beam -> Switch Favorite -> 4. Concussive Shot, Flamethrower, Energy Drain -> Switch to favorite 1 and Repeat.... thats 12 powers and 6 combo detonations all within 15-20 or so seconds, since you bypassed all power cooldowns via favorites switching. Which pretty much broke the game...

And if the issue is they don't want massive spams, they could have made it a bit more... open. Like skills that prime have 1 cooldown, and skills that detonate 1 cooldown. That means playing with favourites would be much more free, and if you cast a primer, no matter if you swap favourite, all priming abilities would still be on cooldown, but whatever other abilities would not be.

I totally understand where you are coming from, and it is unfortunate that a Tech Construct Engineer build (Assault Turret, Unknown, and a 3rd power) gets some what shafted in that way, since I love the Engineer class and would have loved to play that along with being able to self-combo such as Incinerate and Overload.

I haven't played too much with the Tech Construct Engineer build yet because I haven't gotten far enough to unlock Unknown, but I'm kind of holding back because of this limitation. But if I had to change something in-game to accommodate for this, I honestly wouldn't know what to change.

-Your suggestion of primer and detonator powers being on separate cooldowns seems to be complicating things much more than it should be
-Adding a 4th power would be fun and creates even more interesting builds, but it goes into issues with controller configuration and touches a bit on balancing issues/power spam
-Instead of all powers undergoing a cooldown when switching favorites, all powers are readily available upon switching like you suggested but you can't switch Favorites again until like a minute or two. An interesting mechanic but instant power access promotes that one-time power spam but long cooldowns in between like in ME1.

Though personally, just being able to actively control which powers that squadmates use would have been good enough for me. I love using hotkeys for squadmate powers, it truly tests my multi-task and motor skills haha.