Cryomancer
Arcane
Sounds like you are proposing to, guess what, balance them.
Nope. Just make martial classes more interesting. But It is not "replacing the fighter". It is making then more supernatural, just like the wizard has access to supernatural spells, the fighter should have access to a supernatural body and supernatural weapons. On IWD2, animate dead on high levels create undeads with auras and self regeneration. A high level barbarian should be able to have this stuff, combat maneuvers(one of the few good things about 4e), chance to decapitate enemies on melee strike, or mutilate enemies, demolishes walls and buildings swinging his warhammer, move extremely fast, like 120 feet in a round and so on.
On 2e, If I an playing a campaign full of undead, of course a pure roleplay sorcerer that only picks cold and water based spells would have a much harder time than a Paladin which is a holy warrior with a lot of anti undead nasty abilities. But this doesn't means that the "water sorcerer" is not a interesting concept. Most of my builds are pure role play builds.
IT would made martial classes far more interesting. IF the aim is just balance, then remove either the martial or arcane classes would make Baldur's Gate 2 far more balanced. But better? Having a boring "I attack" class is a problem. HAving a unbalanced class is not. For example, pick VtMB. The game is extremely harder as a Nosferatu than as a Tremere. Removing Nosferatu or nerfing the deformity curse into a minor penalty on seduction would make the game extremely more balanced. But far worse. Low INT runs of Fallout 1/2 aren't optimal but are interesting.
The most iconic example is Gothic. I never made a pure mage in Gothic 1/2. Only on 3 and with mods like Returning. BTW, Gothic is not a game where you start with magic. Getting magic itself is a long quest. On returning for eg, many people consider necromancer teh hardest class to be played. But makes sense on Gothic universe.
You need to ask Xardas for apprenticeship. This option only exists on the mod, not on base game, after few days thinking, you become a novice of darkness. Now you need to train your spirit by paying mages to train your mana and train your INTELLIGENCE by doing intellectual stuff like runemaking, book reading,
and etc. After 30 int and 75 mana, you talk to Xardas to do the initiation into the circle of darkness. You must enter the monastery of fire magicians and kill a novice, that quest involves gathering information, planning a escape route and etc and after it, you are fully initialized into the circle of darkness.
Now you need to buy the scroll, the reagents, learn the first magical circle and how to make arrow of darkness rune and make the rune;
and etc. After 30 int and 75 mana, you talk to Xardas to do the initiation into the circle of darkness. You must enter the monastery of fire magicians and kill a novice, that quest involves gathering information, planning a escape route and etc and after it, you are fully initialized into the circle of darkness.
Now you need to buy the scroll, the reagents, learn the first magical circle and how to make arrow of darkness rune and make the rune;
That is not optimal but is very interesting.
Maybe whine more about the conjuration side of things than the necromancer for once,
And one of the worst aspects of BG3 is being 5e which destroyed persistent effects.
For eg?
Pick Black Tentacles. An amazing spell which required no concentration, had a caster levle + 8 to grapple and dealt a lot of damage and compare with 5e tentacles require concentration and are far more easy to fail on grapple the creature. Cloudkill for example, was a amazing spell on BG2 and on 3.5e/Pathfinder games. I loved using cloudkill so much on kingmaker. On 5e, it no longer deals CON damage, requires concentration, and the damage dealt is smaller compared to 5th level evocations. Just 5d8 poison damage.
This are two amazing mid level conjuration spells which 5e ruined.
My guess is that every wizard on BG3 will gonna play as a evoker on BG3.
My main wizard on Solasta was using evocation, the unique school who worths being used on 5e. And BTW, shock arcanist in solasta is amazing. Interesting in mechancis and gameplay. Larian in other hands can only homebrew awful rules.
Except by the AI, there are nothing on BG3 better than in other CRPG's.