Balancing turn-based combat really is insanely hard because there are so many factors to control for, including player builds and ability, promoting variety of combats, exploitative player behavior, etc. I do hope the hardcore mode (pretty much confirmed to be called this in the official "Creating Combat" document in the mod section of the forums) also includes certain AI improvements. I think people have laid out the problems pretty thoroughly, so I'll just compile what I think would be good solutions, some of which have already been mentioned here or elsewhere.
Nerf leadership bonus down to 3/6 or 2/4. Making iniative a roll instead of a check could be interesting as well, especially if it happened within rounds as well. So archer and rogue enemies would tend to move first within a round since they're high perception and speed generally, but a mage might go first when you least expect. In general randomness doesn't necessarily make games harder, and can be frustrating, but I think in this case it could work well, because you wouldn't be able to predict exactly how enemies will go in turns so easily then.
Increase sightlines of many enemies immensely so it wouldn't be so easy for a mage to stand at the edge of their vision and get an easy first strike. Another solution would be for a player who uses a spell to strike inattentive enemies to lose all of their initiative. I found it stupid when you used a spell and then combat starts, and you can cast another spell immediately after because you won the initiative check.
Nerf charm hard. Make it last 2 rounds, max, and make it so enemies do not target charmed enemies, and will in fact try to cc them if they can. In turn, you should be able to completely control a charmed enemy, so you don't feel jipped because they did something completely useless.
Nerf CC in general. Make it harder to apply CC and make them last shorter.
Reduce health of summons, increase speed, and give them stench. Basically, what the source difficulty mod did works really well (though not sure about giving them comeback kid like the mod). Summons should be flankers and opportunists, not tanks.
Massively reduce resists found on gear and capable with crafting.
Give enemies more spells and more if-then checks to help them determine when to use certain spells or abilities. So they should try to use ricochet only when there's at least three targets grouped together. I know this is quite complex, but I do think they could do a few things regarding AoE spells and avoiding healing your summons with the wrong spells and the like.
There's an interesting discussion here about whether enemies could take advantage of loremaster, if they should "learn" about player resists and immunities and the like, or if they could at least take advantage of "common sense" that fire elementals are healed by fire damage, for example. I think loremaster could be slightly reworked so that someone with 1 or 2 loremaster would be able to see the fire resist of a fire elemental, the water resist of an ice elemental, and obvious cases like that. But they wouldn't be able to know the player's resists unless they had higher loremaster or if they realized a spell did little damage to a player. Giving certain enemies high loremaster would be an interesting mechanic, though I kind of doubt Larian would go to the trouble to do this. A behind-the-scenes check for some or all enemies would probably be the easiest solution to eliminating stupid AI decisions.
Sneak should not go down to 1 AP. You should not be sneaking and attacking several times in a turn. Perhaps players should only be able to sneak once in a round? There is a complex balancing act to be had here, and I need to play a bit more rogue myself to really offer better suggestions, but from what I've seen and heard, rogues are kind of broken. They either die really fast or do absurd amounts of damage. I just feel like they don't have enough abilities. I think Larian should figure out ways to take advantage of pickpocket in combat, like if you could pickpocket a health potion and replace it with a poison potion, for example. Enemies would do perception or loremaster rolls or something to decide if they should drink the replaced potion or not. It would also be cool if rogues could collect traps with the lockpicking skill and place them later. Overall, I think rogues need to be fleshed out more and given more abilities.
Almost every single talent needs to be buffed or nerfed. Most are useless, and some are insane, and just a few are in the sweet spot. There's lots of room to make interesting talents that aren't over the top.
Most of these ideas are number tweaks, because as Blinkicide pointed out, making predictive AI that doesn't take a while to calculate is insanely hard. But let's start with the easy changes before we get into more theoretical programming. These changes together the game would be incredibly different and challenging. There's tons of things Larian could do rebalance the game, and I'm excited to see what they're going to do.