By the time EKs get Fireball at level 13 it's probably not as gamebreaking. I guess the "power build" in BG3 could be a Sorcerer 9 / Fighter 2 that could erase all encounters with an alpha strike before they even get a turn.
That's true, I didn't take that into consideration; I just used Fireball as example because it had been mentioned before.
It seems likely to me that extra attacks will be implemented as extra actions (just like the extra attack from Haste is implemented right now)
Really? I expected this to be implemented by adding ability that will do two strikes instead of one ...
(BTW i was unable to find it, but should we be able to attack two separate targets with multiattack or not?)
Yes, I think so, because of how Haste was implemented as just giving you another action "point" to use. I can be wrong, of course, and we'll see how they do it when we see it.
And yeah, they should be able to attack different targets or even move between attacks if they want to. From the Player's Handbook, Chapter 9: Combat, Movement and Position, Breaking Up Your Move (just getting all the subtitles in there since I can't give you a page number

):
Moving Between Attacks
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
action, action, Surge, action, action, Haste action
I dare to presume Surge also restores only one Action, just as it does now (doesnt it? didnt try on patch 6) while under effect of potion.

Depends entirely on whether you or I turn out to be correct about whether Extra Attack will be implemented as an extra attack or another action, I guess.
limit it to one spell per action type. So a Hasted wizard could potentially Misty Step (bonus action) away from melee, throw a Fireball (action), but then not be able to throw a second Fireball with his Haste action but have to lob a cantrip at the enemy instead.
I cant quite imagine how would you like to explain this rule to new players.

I would explain it like I just did. When a player tried to use another spell with another action type I would just have a red text notification appear that said they couldn't and why, just like how it does now when you can't move rocks and so on.