Won't happen because

Every suit would need 4 resistance types, 4 ammo types for the 3 tiers of weaponry, new skills, new slots on suits to carry new ammo types and or switch between them. Crunching numbers for different suits to have different base resistances, new skills to alter these resistances, new modules to alter these resistances, passive hardeners, and then the active hardeners your proposing.

Considering Ccp can't even put my amarr character in the correct merc quarters (it's a gallente room btw) their entire company would implode trying to implement that.
Lives will be lost.

I'd rather see vehicles and suits have capacitors and teach people how to run active mods on a stable cap suitor they power down mid gunfight and die. cooldowns just encourage tanks to retreat to the redline and camp for 60seconds.

Lets look at some math just to see how challenging this would be to balance and implement well. My initial thoughts would be that there would be some suits this wouldn't make sense on and some suits that would be dumb to then fit anything else other than hardeners.

Let's say hardeners added 60% hardening. So if a complex shield module is 66 to 73 HP depending on skills and your base shield HP on your suits are something like:

Lights: 70-125
Mediums: 90-225
Heavies: 405-506

A hardener would add more eHP @ 60% for unskilled shields at HP as low as 110. So, basically anything but an armor scout would benefit. The heavy's huge intial pool of HP would benefit the most to these hardeners by adding 250-300 eHP. This is much more significant than plates.

If you could stack two with the normal drawbacks, you could get over 1000 eHP on a medium basic amarr heavy suit's shields. Caldari medium frames could get 700-900 ehp pretty easily, depending on fitting cost.

If these modules didn't give a shield delay drawback like the shield modules themselves, they would likely be much better than the regular shield extenders in practically every way. However you could kind of justify them by saying they are kind of like a shield delay in that the 'cooldown' time is a period of time during which you do not have your full shields.

Alternatively, if you think about this from an armor perspective, are many suits that have a lot of low slots and even higher base armor would get even higher eHP unless the hardeners there were less. However stacking armor hardeners, especially if they lasted more than 15 seconds would potentially mean that the effectiveness of repairs would skyrocket. The proto 125 hp/s repped would multiply with 5 hardeners @ 50% to something over 600 HP/s I believe. Multiple reps would mean that heavies (or anything with 4-5 lowslots) would practically be invincible to light weapon fire during their hardening cycle and while receiving reps. These might be difficult conditions to satisfy in many instances, and could be countered by prioritizing fire on logis vs. heavies, but you see what would start to occurr.