I think the customization options just put a lie to the argument that 13th Level is game breaking. You can add +4 proficiency bonus with custom settings. A 13th Level Fighter has 7% more hp, +1 proficiency bonus, and can use indomitable twice per day instead of once. Wizards can throw an additional 7th Level Spell, but a heightened fireball is +1d6 damage and a Delayed Blast Fireball, if you Concentrate for one Round, is +2d6 compared to 12th Level (once per day). Teleportation was a 7th Level spell that changed the game in TTRPG, but it would have minimal impact in BG3. 13th Level characters are certainly stronger than 12th Level characters, but not dramatically so. And if you choose the custom option to start with a -1 Proficiency Bonus (instead of +0 to +4), it barely moves the needle.
I would like Larian to open up the Level Cap and add a few more customizations that make the game harder. Wandering Monsters, especially if they attack when you are sleeping in an unsafe area. Monsters that aggro when you make noise nearby. Roaming monsters that have a much larger range than the placed monsters and that try to run away to team up with other monsters if they are losing. Traveling monsters that run to another map if they can get away (like a Blighted Village goblin that runs across the bridge to let the next goblin village know there is a goblin slaying party to watch out for so they aggro when they see you). And allowing monsters to have the dying condition so other monsters can aid them just like PCs can aid their allies. Roaming monsters that set up an ambush when you try to leave an area you thought was cleared coupled with no fast travel except with the Runes. Also, in some cases, turn placed monsters into a roaming warband if they aggro, potentially teaming up with allies, rather than returning to their placed positions after they aggro. I understand the placed positions are chosen for strategic effect but if you attack from stealth repeatedly, the strategy already failed, so make those monsters roam.
Monster options: Add more acid flasks or AoE spells that monsters throw into areas of darkness, the place from which they were just attacked from stealth, or where someone just went invisible. Give Wizard (or Ranger) NPCs the Alarm Spell strategically located. If PCs attack from stealth within 30', almost any 3rd Level spellcaster can have Locate Object (and can lead their friends inexorably to the attacker). Increase the roaming range of monsters that cast Invisibility, so they search everywhere they can once they aggro. Meld into Stone after a monster goes aggro and before they stop aggro because they can't find you (that lasts 8 hours). Clairvoyance at the place from which a monster was attacked prior to stop aggro (lasts 10 minutes) in case PC goes back to the same spot or Arcane Eye that looks for PCs in a very wide range (for an hour). And how about Divination (from where will we be attacked?) that lets the AI try to set up an ambush if you strike from stealth a second time.
Also, increased XP to level up on harder (or custom) settings to (at least) account for the wandering monsters.
I'm not advocating taking anything away (other than fast travel) because I'm sure some people like the modifications, like throwable healing potions. I'm just suggesting ways monsters can be much more effective without fiddling with the 5e (or BG3) rules.
