Actually they can fix it easily at least in stat sense.
- Half amount of monsters per map and packs shouldn't be bigger than 5-10 monsters (more stragglers less packs)
- Double or even triple monster HP (bosses HP is fine though)
- Reduce monster damage in half (leave bosses and rares out of this nerf)
- Buff damage by 50% single target melee skills and give ancestral call/splash 50% reduced damage
- Make bosses actually have way more % to drop something good than now
The whole other thing is melee combat need huge animation redesign.
Adding ragdolls also would be great.
I love how Wolcen(known earlier as umbra) basically 1ups everyone when it comes to melee combat and shows that GGG doesn't have anymore any excuse to not make combat actually feel good. Their previous excuse was that they are not big dev compared to Diablo and we accepted that but this was in 2012 not in fucking 2018.
Now Wolcen is done by amateurs with small team and combat looks like this:
At this point you're talking entirely different game.
And casters/ranged would STILL be better after your proposed changes:
1) ranged means no need to waste time to get close to target.
2) ranged means easier time targeting enemies.
3) ranged means never having to deal with enemy melee skills unless you're retarded and forced yourself into a situation where you can't kite.
4) ranged usually has easier time scaling high-end damage without sacrificing too much of other build aspects compared to melee. Even increasing (non-rare/unique) monster life 10x will not do much to deter the current ranged build meta while seriously crippling melee.
More time to kill mob = more time under attack from the mob, remember? Except that ranged builds will NOT be under attack due to not being in range (or easily avoiding ranged attacks).
5) 50% more single target solves nothing because aoe is king. You mentioned noting about multiproj skills (remember - EVERY proj skill is a multiproj skill because there are ways to make it so. All aoe has is crippled aoe scaling. Melee? Hahaha, melee has NOTHING).
6) buffing boss loot to anything remotely valuable will simply lead to new meta where community quickly discovers IF and WHEN bossing is valuable, then proceed to farm that.
Case in point - shaper strongholds, a fresh 3.4.0 addition.
Ragdolls will never happen. GGG has discussed it numerous times. WAY too much strain on the engine for basically zero benefit to the game.