So once you know the story what makes us replay the game over and over again ?
Mostly... The different dialogue options available to different classes, races and skills.
They can provide alternate ways to interact with the story. More so if choices have actual consequences (This tends to not be much of a thing in BG3 though...)
For your next game im a bit worried because a lot of the fun things that make BG3 so replayable, combat, these types of itemsets, dnd spells and level progression are all there because of this game is based on DnD ruleset
Ehh... None of that is specific to DnD. Heck, it can be argued that Larian adhering to DnD actually made these things WORSE than if they had their own systems.
The turn based combat is nothing new. It's actually staple to the genre. Larian's prior game, DoS2 also had plenty of interesting encounters. Though, hopefully they've learned that the crappy armour system from DoS2 is not good or fun design.
Unique items are also nothing new. Again, another staple of the genre. Games like Pillars of Eternity (1+2) as well as Larian's prior titles (Divinity 1+2) have plenty of cool and interesting magical items to create builds around (Even more so DoS2 where mod support allowed for a number of mods to be created that added even more magical items, incorporated in the base game's "Treasure Hunt" style where they were hidden about the game making exploration fun and interesting). With Pillars Soulbound items being the most interesting, providing you with tasks to complete to upgrade them into more powerful versions.
Spells are nothing new. DnD spells aren't unique in their creativity, especially BG3's hamstrung versions (Because of trying to translate DnD mechanics into a video game, many spells simply are garbage compared to their TT iterations). Larian's prior title DoS2 had plenty of cool and interesting spells and also additionally, plenty of cool and interesting interactions because of their surfaces fetish (Many more surface interactions and of course the Blessed and Cursed modifiers). While I'd say that honestly Pillars of Eternity 2 had way more interesting spells (And mechanics with the innate Spellshaping ability) than BG3, with exception of the OPness that was the Toilet Combo for late game Wizards (Pull of Eora + Stacked persistent AoE damage spells like Wall of Flame, Ninaguath's Freezing Pillar, Meteor Shower, Storm of Holy Fire etc) which basically trivialized every single combat by constantly pulling and slowing enemies into the damage.
Level progression in BG3 isn't unique either. Nor would I consider it to be a particularly good iteration. Personally, I think DoS2's abilities are more interesting than the 5e's Feats (Even more so when you consider just how good ASI is in DnD... Getting +2 in a stat for +1 on EVERY SINGLE ROLL that uses that stat is ridiculously good). DoS2's skill increases are also interesting with their passive bonuses on top of their ability unlocking. While I also think that Pillars of Eternity's attributes are the best in the genre because ALL of them are useful for ALL characters making for more depth to character building.
The one caveat I have with PoE's attributes, is their names and thus associated characteristics. For example, my Wizard stacked Might to make her spells more powerful... But this meant she was called upon to do feats of strength like throwing herself into walls to smash them down, or ripping doors open. While my Barbarian stacked Intellect to make his cleaves hit in a wider area allowing for tons of AoE damage as he's whacking away in the middle of a pile of bad guys... But this meant the dumb "I hit it with my axe" character was always the one providing knowledge insights...
While you can design other types of combats and spells and level progression, BG3's DnD system is by far superior to anything that has been seen in these types of games so far
Honestly, I'd argue that BG3's DnD system isn't even on par with prior DnD systems, let alone many non-DnD systems.
BG3's watered down version of DnD (Of 5e DnD no less, the more watered down edition of DnD itself...) is not good. There's so much depth and creativity that is just lacking in BG3's systems as well as plenty of things made stupidly powerful for no reason.
Not to mention the benefits of reusing the design, no need to spend so much development on spells and level progression, that allows you to spend the time and money to design something more like higher level cap or something like "mythic power" system as in wotr that you can basically invent completely by yourself.
The level cap was done specifically BECAUSE of the DnD systems. More accurately, because "Higher level spells and abilities from DnD don't translate very well into a video game" that is the reason why the levels in BG3 were capped to 10 (Later upped to 12). Continuing to use DnD systems won't suddenly make it more viable to include the higher levels, it will continue to have the issue of higher level spells and abilities simply not meshing well with a video game and thus will continue to need either capping of levels or more deviation from the DnD systems itself.
Something like "Mythic Power" is a curious thing to wish for when praising DnD systems, because it is outright ignoring DnD systems and being designed as an independent thing. If you think that Mythic Power was fun and interesting design, then you're showing that non-DnD systems can also be fun and interesting design.
But combats are little bit too easy for veteran players and thats why i would like to have option in difficulty settings where number of enemies could be increased little bit to increase difficulty, maybe the extra exp problem could be solved easily so that extra enemies that you get using this option, doesnt provide any experience so character progression speed stays the same and you wont be lvl 12 at kethric.
Honestly... Most other games in the genre already add additional enemies with higher difficulties. Owlcat's games all do this (Kingmaker, WotR, Rogue Trader), if I recall correctly Divinity did this, as well as Pillars of Eternity.
As far as experience goes... I'm personally of the mindset of wanting experience to be removed from combat altogether. As well as other things like skill usage and side quests. I'd really love to see games put experience entirely into the main story.
This way, you have more freedom in how you approach the game. You don't have to run around killing every enemy and doing every little quest and ensuring you do maximum skill checks simply because you need to suck up all the experience possible. Thus you can do things like sneak past combat encounters, bypass skill checks using creative methods, do only quests that it makes sense for your roleplay (I.e. No saving people on a Chaotic Evil playthrough or no theiving/murder on a Lawful Good playthrough)
It also would provide more freedom in game design too. You can add in more content, more (Potential) enemies, more methods of solving problems, all without having to worry about balancing experience gains or people over/under leveling depending on how they play the game.
In addition, you can more easily tie increases in power to the story itself. If you perform a heroic deed in the story, you get a level up as a result. Some crazy shenans happen and you absorb some magical power, a level up. This provides a better narrative to your rise in power than "I killed some rats... Now I can summon God to smite my enemies!"