Combat in ME3 was actually quite good, imo.
I like Tech and biotic combos, but there are really to many detonator powers. Maybe only warp should be per default able to detonate warp combos, while other powers could get an evo that also allows that.
For tech explosions, maybe not let them work directly on (all) enemies, but on specific objects (with a bigger radius then). AKA (ME2 similar) A flammer trooper hit incinerate->FE, Cerberus shield-pylon hit with overload-> TB or similar.
Personally I like Overheating weapons better than Thermal clips. This was something that set ME apart from the other shooter hybrids out there.
As for thermal clips versus weapon overheat: I am a huge proponent of thermal clips, if only for the simple fact that is one way (among many) to increase the variety of weapons, and the only real way to make all weapons balanced and viable.
Allow me to explain by using shotguns as an example by comparing the Claymore and Scimitar.
1) The Claymore works as a weapon since it has only one shot per clip, generally enough to kill one enemy before you have to reload. Now compare that to the Scimitar where it will take a few shots to kill one enemy, but then you can go kill another enemy without needing to reload.
2) Now how would the Claymore and Scimitar work with an overheating mechanic instead of thermal clips? You fire one shot with the Claymore and it overheats? Since all overheating has the same duration, the Scimitar is now the only viable shotgun since you can kill more enemies faster with it, whereas with the Claymore you can at most one enemy (assuming you hit) and then have to wait.
The overheating mechanic essentially turns all weapons into a DPS (damage per second) race, and we are back to ME1 where you only have one weapon per category.
Not necessarily. It could also turn more into a sort of Battletech/Mechwarrior style weapon builds. (If they keep per weapon max. RoF)
Say, The Claymore and Scimitar both vent of a similar amount of Heat per second.
-The Claymore does more damage and generates more heat, nearly overheating when shot. Cooling takes X seconds, similar to reloading.
You can also fire 2 shots and have to wait 2X seconds until it is cool again.
-Scimitar does way less damage, but also generates less heat. You can fire 5 shots one after another and it nearly overheats, then also wait X seconds.
You can also a 6th shot immediatly, also having to wait 2X seconds until it is cooled down again
Or you can Fire a single shot and wait X/5 long until it is completely cool.
What you got is a different Damage, Rof and Rof+'reload'/cooldown behaviour per weapon, similar to what we have now, minus reloading.
Add some weapon mods to that, so the shotguns can be more damaging, or cool faster, or have a bigger heat-capacity.
Then you have your never ending fire Scimitar, or a OHK-everithing Claymore that overheads after every shot. OR something inbetween, e.g., a 2 shot, lower damage, claymore with faster Rof than the wraith.
The maximum Rate of fire achievable for each gun would be the minimum of (heat_capacity + cooldown per second)/generated heat and the specific weapons hardcoded RoF.
Bring back the defence modifiers, and your 2 shot Claymore is good against lone, shielded or organic enemies, the scimitar against multiple organic or low shielded enemies, and the 1shot uberClaymore and the slow high damage wraith are not bad against armour, too.
Of course, certain mods would better fit to certain weapons, like fast fire weapons and cooling mods, or slower weapons and damage mods.
Certain configurations will end in weapons behaving similar, but this is already now so. And stuff like Widow/Black Widow wouldn't need an extra entry somewhere.
Also Bakaras 2-Shot Claymore would be possible.
Edit: About some people turning this into a DPS race, that has not changed with the introduction of Thermal clips.
Who really plays MP with the Katana or Phalanx anymore?
Heat management would actually broaden the use cases for these guns, if done right (might be a balance nightmare though).