Giving sorcerer +1 spell per level will neither break the class or make it overpowered.
It will only give it the bare minimum versatility it needs to utilize its spell focuses.
Sorcerers have a pitifully small quantity of spells that that they can memorize at each level.
Most of these must be dedicated to memorizing mandatory spells that every caster must take, after which: You're left with virtually no room for customization to speak of whatsoever.
If a sorcerer bothers with spell focuses at all (aside from abjuration), they quickly discover that they have very little room to actually select DC spells that utilize their spell foci, unless they're willing to make huge, very punishing sacrifices to their build.
Sorcerer is so egregiously unpopular that any sorcerer you spot in-game may as well be called a unicorn. That in itself speaks to the current, sad state of the class.
It has been heavily nerfed, even more so than other DC casters. And in its current state, it is not the game-breakingly terrifying monster people make it out to be. Giving it an additional spell per level would not make it such.
Divine sorcerer has already been specifically targeted with significant nerfs, particularly to its AC: Its primary defensive attribute against melees. Its AC is nowhere near untouchable or overpowered at current. Hence why even divine sorcerers are unpopular unicorns in the current meta.
- A 26/4 or 27/3 divine sorcerer right now can self buff 55 AC with EMA, divine shield, haste, mage armor, shadow shield, and cat's grace (x2) with a base dexterity of 8, which is standard. You can shove your AC up to 56 with a red ioun stone and 57 if you rune your gear for +2 dexterity. And if you manage to fit armor skin into the build (at the cost of either Hellball, GR, an Epic Spell Focus, or the mandatory feats of ESF Disc + ESF Concentration) then you can push it to 59 at great sacrifice to your build.
- To put this into perspective, the average melee strength build is walking around with 51-52 AB not including true strike or buffs from their party members. Self-buffed, they need to roll a 7 or higher to hit 59 AC without truestrike, which is absolutely trivial.
Sorcerers should be the least of your concern. DC casters are not the top of the food chain in 2023 Arelith.
All DC casters on Arelith have been the recipient of extensive nerfs to their offensive capability (both their DC spells and their summons). It's WM and Rogue summer, not DC Sorcerer/Wizard summer.
Reason #1: Spellcraft now applies versus all spells (including 9th level)
- This is the bandaid that made it so that even classes that traditionally get horrifically bad saves (ex: Weapon Master) can now achieve practically untouchable, spell-impervious saves with a mere bit of gearing investment and spellcraft dump.
- Before this change, melee builds had to attentively ensure they kept up their death ward, clarity/mind blank, and freedom. Where as now, if you know what you're doing, you can safely run at a mage without using any of the three, knowing your +6 to +16 unisaves from spellcraft will compensate for what might otherwise be one of your build's most significant weaknesses.
- Normally in NWN, it was the paladins, blackguards, and CoTs who were the most feared anti-mage melee builds in the game (barring SR monks) because their saves were so high that they simply could not be hard CC'd with DC spells. Now, virtually any melee build can boast that title if they but gear a little bit for saves and skill-dump into spellcraft.
- The only builds that still have bad saves versus spells in the current meta are ones played by people who either do not care, or whom have been grossly negligent in their gearing.
Reason #2: Sequencers no longer protect summons from dismissal via SR or Mantles.
Due to this change, anyone with access to Word of Faith, Banishment, or Dismissal scrolls - or the spells themselves) can instantly remove a mage's summons from the battlefield with minimal effort. Which means they're dismissible by just about everyone.
IN SUMMARY:
DUE TO THE SPELLCRAFT CHANGES:
- High 30s to low 40s universal saves are now easily achievable by most anyone who makes the effort, no matter how saves-deprived their build might normally be.
- Consequently, properly-built melee builds are now nearly impossible to hard CC or pin down with DC spells, which is enormously bad news for the average 390-420 HP DC caster.
- Traditionally low saves melee builds are no longer low saves builds, as they once were, because they readily have access to up to +16 universal saves via spellcraft. (33 base, +2 on 11 items = 66 spellcraft = 13 to all saves. For those who really try, add SF + ESF for 3 more for a total of +16 to unisaves.)
- Bad saves versus spells now only exist if a player is simply grossly negligent, uncaring, or incompetent.
- Competent players will always stat even their weaponmasters to be virtually immune to DC spells.
DUE TO THE SUMMON CHANGES: Barring built-in resistances to WoF/Banishment/Dismissal, summons are a non-factor in PvP against competent players. They cannot be relied upon for viability in PvP.
ON THE TOPIC OF AN EVOKER'S POSSIBLE DAMAGE OUTPUT: Most evocation damage spells are reflex save for half, which means about 60 damage. 120 damage if casted twice per round.
ON THE TOPIC OF IGMS: Maximized IGMS casted twice per round does 240 damage. You can outheal that and then some by simply mindlessly chugging heal potions until they run out of spells. Furthermore, SF abjuration and shield potions are easily attainable for immunity.
A MEELE BUILD'S DAMAGE IN COMPARISON: Melee builds frequently can achieve up to 50-80 damage per hit and crit for 110-200+ damage per hit at 5-6 APR. They SIGNIFICANTLY outdamage Sorcerers and Wizards unless they neglect to dispose of their summons, which they can readily do with a single scroll.
DUE TO ALL OF THE ABOVE, each of you should be a lot less worried about +1 spell per level on Sorcerer, and a little more worried about the melee build that can dish out 600-700 damage in a single round.
The OP's suggestion of an additional spell per level offers mere quality of life customization to a currently underplayed, unpopular class that has been shoved out of the meta due to far too many passes with the nerf-hammer.
It will not break sorcerer or make it overpowered. Relax.