As far as prebuffing, I admit I did this all the time in Baldurs Gate and Icewind Dale. I would hide in shadows and scout the areas as far as I could, and then stand just out of sight and buff up the whole party if the fight looked difficult. Especially for boss fights,I would guzzle down dozens of potions too before combat. I don't see a problem with it, as long as you're not already in combat. So, I would definitely use prebuffing in KOTC frequently before opening certain doors.
It does not follow that enemies should be prebuffed just because the party can be. Unless the enemies know they have intruders, why would they prebuff beyond a couple of Day duration spells like Mage Armor? It's a question of whether or not they know you're coming. If the enemy know you are coming, from an alarm or a perception check vs noise, then sure they would prebuff if possible. Otherwise, they would not. And note if they hear you just one or two rounds away, that's all the time they have to prebuff! Same for the party. If they get caught surprised, no prebuff for sure.
I would like to see many more support and utility spells and other non damaging spells to be assigned "Move" and "Swift" and even "Free" action costs, and many more to have Day long duration. I think it has already been suggested to make long duration spells just be Day long duration. I like that. They already cost a spell slot. That is enough. Personally, when I only get a few spell slots and can't cast before combat, and they essentially take my whole turn to cast, i rarely cast buff spells during combat (with a few key exceptions from time to time). A whole turn just for a +1 TH? Not worth it when I can do some damage or summon a creature. I would cast it outside of combat though if it lasted long enough for more than one encounter. I like the suggestion made in another post that if they must be cast in combat only, to increase the effectiveness of all the buff spells, and to also increase the number of Mass spells that affect the whole party or all enemies. Maybe it could be a game option to make all non-attack spells become Move actions to cast?
As for multiclassing, it can be very cool and I have made quite a few F/M/T in my time, although that was mainly in old AD&D where there were few classes and no feats really. But in KOTC 2 there are plenty of classes and feat options, and the party size is large enough I don't feel a need for multiclassing - EXCEPT, I would really LOVE to see Prestige Classes or Kits added to the game in some shape or form. I do find that extremely fun for a bit of specialization. I would like to see it implemented as just an extra pool of related class feats you have access to rather than as a multiclass approach, and allowance for extra "Prestige" or "Kit" feat picks or automatic progressions every level on top of the usual feat picks. Maybe even just implemented as class specific spells like the way the Rogue currently works. So, if you choose to take Arcane Archer, then things like Enchant Arrow and Arrow of Death are added to your valid Feat/Spell list. No one else can get these feats/spells. So, really more like a Kit in the AD&D sense, since a few benefits and restrictions might get added to your main class. Pale Master was another favorite of mine! This being said, I wouldn't mind just seeing a bunch of new classes added that are named by the Prestige Classes. So I could just plain be an Arcane Archer or a Pale Master right from the start. Actually, that might be even better