First of all, thanks for the detailed reply and the breakdown. To address the first point, i'm aware of other builds but even when you include the other faction specific builds rather than just grineer, the variety isn't really there and it comes down to swapping the elemental mods most of the time. So i just was thinking of a system that would make it more interesting. The most i go for is SP so i don't really fight with 200+ level units but brute forcing with corrosive or viral/hm gets the job done most of the time so i don't even bother and looking at other builds on youtube or overframe, this seems to be the case most of the time. Eximus and other special units have extra damage reduction methods so they require a specific build to be dealt with in higher levels but i was talking about the general modding rather than min-maxing.
Singular elements can be good but corrosive and viral is far better in practice than most others, you really need a special case related to the gun's gimmick like guaranteed impact to use hemorrhage etc. So the elemental effects need some kind of overhaul as well, something like reaching certain number of stacks trigger different effects. For example:
<10 stacks of electricity: target is stunned for a short time and resets their animation/attack (excluding special units)
>10 and <20 stacks of electricity: enemies near 5m of the target is also stunned for a moderate amount of time
@20 stacks of electricity: causes a small burst of electricity wave jumping from target to target, gaining more damage with each jump, hits up to 10 target
This imo is an interesting way of handling status effects and this can be further modified with mods that alter these effects to customize gameplay. Imagine you have a rifle that uses a mod that buffs max stack effect so you need to constantly apply status with high firerate, and compare that to a mod that buffs the initial (<10 stack) effect of the status which rewards instakill or high damage weapons. These can be elemental or utility mods.
As for the questions;
I initially removed exilus slots and replaced them with utility slots but after that decided the weapon specific utility mods could be left as exilus mods such as ambush optics for rubico. Conditional mods or ammo drum etc. can be utility mods for example, they can be classified as mods that do not directly increase the damage related stats of the weapon. Adding new weapon specific mods which are not just boring stat modifiers could add more exilus mods and make better use of this system.
Actually yes, it limits the player but i think it does it in a good way that requires you to change the go-to default builds. In turn, the mods can be buffed slightly or the new extra slot can be used. As opposed to 8+1(exilus) slots for the weapons we currently have, this system has 10 slots that can be arranged in different ways, if it has like 6-7 damage slots, 2 elemental and 1 utility slot, that would be more powerful than the builds we currently have. It all comes down to the slot configuration.
I think this system can work on its own since in practicality at least half of your build will be the same, but the extra slot and mod overhaul (of generally weaker mods) could make modding really interesting. If less used mods are ever removed from the game or combined with other mods to create new better mods and make the overcrowded mods screen less crowded, making every similar/related mod a set mod can be even better. Set bonuses are already a thing but they are on lesser used mods, rather than main damage/elemental mods. So that can be improved as well, might make you use a similar mod with lesser value just for its set effect.
This idea in its core belongs to Brozime, i had thought of a similar thing with slot types but he put it into words first and i just changed it a bit and shared it here. With more suggestions and ideas it can be better of course but right now i am as a player not motivated to open the modding screen because of the reasons listed above. Even without the default go-to builds, experimenting is discouraged with forma being a frustrating mechanic. I want to try a different build just to find out that the polarity needs to be changed... Then i close the game. At least it should be possible to temporarily forma in simulacrum to test builds.