That would be terrible and would really hamper any sort of fighter who wanted to do any damage.
Currently a fighter needs high resolve and perception in order to get a good deflection, cause interruptions and defend against interruptions via concentration. To do decent damage you need a high dexterity and might. This leaves intelligence and/or constitution as dump stats to fund the stats that you think are more worthy. Dropping intelligence drastically reduces your durations but is a fair trade off. Dropping constitution to high single digits, 7-9, is a calculated risk, you'll have less endurance but you can survive by being either having more deflection or doing more damage faster.
Your proposal would force fighters to pump up constitution or else be dropped as fast as a wizard without the benefit of staying safely in the rear. The only place to get the points would be from dex and might, the two stats that increase your damage. This would force fighters into being only damage sponges. The effect on glass cannon casters would be negligible or even give them a boost depending on the default starting endurance level.
The ranged classes already have a big advantage in the game, no reason to nerf fighters who try to do decent damage while still being able to survive in melee.
The proposal of having constitution reduce armor penalties would be interesting if it was comparable to the speed increase from dexterity. A fighter could then drop dexterity in favor of constitution to maintain a balanced tanking/dps or drop the tanky stats in order to go as fast as possible. This would open up many options for builds.
But Isn't that the point? If you want a heavy hitting fighter get an avg constitution of 10 or so, if you want a tank that soaks damage like a sponge pump it up to 20 and risk not being able to spread your points into other stats to do stats.... seems balanced
You could always play with the numbers to bump up the base constitution so its not so unforgiving, but I Believe something along these lines would be useful. Plus it kinda makes since a high constitution character will have lots of health vs one that has little.
endurance = level * constitution points * someSmallMultiplier.
Maybe wizard someSmallMultiplier = 1.0
A rogue someSmallMultiplier =1.5
Fighter someSmallMultiplier = 2.0
This way the more hardy classes will be more hardy because they will get more endurance/health per point of constitution.