semangat gays :v
opening choice for 1900 Fide
You seem to like aggressive stuff - why not try the Black Knights' Tango? It's not very well known but if you enjoy the KID I think it is something you will like. Here is a GM game:

It looks interesting but I am looking for a "main" opening, adapted to my weak level.
Queen's Gambit, Nimzo, Grunfeld, etc ...

Also, as a second opening on e4, I am interested in the modern but feel like it is too complicated for my level as well
The Grunfeld is hard to recommend, even though black is fully equal in all lines, because of the ridiculous amount of theory. You can't go wrong with the Nimzo-Indian, but you need something against the Anti-Nimzo-Indian and Catalan, nonetheless this is what I'd suggest
Second opening against e4 - I'd go for the Caro-Kann because it can be played intuitively. I played the Caro in the past and black is perfectly happy in the main lines, though the sidelines can be fairly irritating

Since you have already played the Benoni- what is wrong with a Volga (AKA Benko) gambit based repertoire?
The Volga/Benko is sound, much easier to play than the Benoni, and while it does have some theory to learn, it is not too extended.
Vs. 1.e4: keep playing the Najdorf. It does take a few years of hard work to master it properly...

Hello,
is the Benko Gambit viable against strong opponents in long games?
I want to continue playing the Najdorf of course but I would like to have a second "weapon" against e4 as well
I didn't know the Grunfeld was very theoritical btw!
In benko compensation is based on positional gain so it is hard nut be cracked.
Against 1e4- why not try Scandinavian defence?

Hello,
is the Benko Gambit viable against strong opponents in long games?
I want to continue playing the Najdorf of course but I would like to have a second "weapon" against e4 as well
I didn't know the Grunfeld was very theoritical btw!
Yes, the Benko is entirely reliable. A plus is that there are several good works published on it- no need to go into details right now.
A quick and clean second opening against 1.e4 is the 3...Qd8 Scandinavian, which is solid, with little theory to learn, and basically "one- and-a-half" key pawn structures to master. Published material there is a mediocre book, and two courses by IM Bartholomew in chessable. You can pick any of these two, although I feel that the free course is not inferior to the paid one.

Hello,
is the Benko Gambit viable against strong opponents in long games?
I want to continue playing the Najdorf of course but I would like to have a second "weapon" against e4 as well
I didn't know the Grunfeld was very theoritical btw!
Yes, the Benko is entirely reliable. A plus is that there are several good works published on it- no need to go into details right now.
A quick and clean second opening against 1.e4 is the 3...Qd8 Scandinavian, which is solid, with little theory to learn, and basically "one- and-a-half" key pawn structures to master. Published material there is a mediocre book, and two courses by IM Bartholomew in chessable. You can pick any of these two, although I feel that the free course is not inferior to the paid one.
Then why not the Caro-Kann instead? The same structure, slightly sounder, much better study material.

Hello,
is the Benko Gambit viable against strong opponents in long games?
I want to continue playing the Najdorf of course but I would like to have a second "weapon" against e4 as well
I didn't know the Grunfeld was very theoritical btw!
Yes, the Benko is entirely reliable. A plus is that there are several good works published on it- no need to go into details right now.
A quick and clean second opening against 1.e4 is the 3...Qd8 Scandinavian, which is solid, with little theory to learn, and basically "one- and-a-half" key pawn structures to master. Published material there is a mediocre book, and two courses by IM Bartholomew in chessable. You can pick any of these two, although I feel that the free course is not inferior to the paid one.
better not buy lowinger's book?

If you want a second opening against d4, consider playing the Dutch Defense. The Leningrad Dutch is similar to the KID, except you already have f5 in! Against e4 I play the Pirc Defense, which is a fighting game from the first move. Both of these openings are ultra-aggressive, so make sure to use them as surprise weapons, since they'll drain all of your energy.

@Pfren gives some good advice!
It sounds like you've been following Topalov's games judging from your current repertoire. Take a look at some of his other openings if you decide the Benko isn't a good choice. He's been known to play Slavs occasionally too. That's quite a different pawn structure from what you're used to probably, and does require quite a bit of work. But it does offer good dynamic chances.

I really dont like the Scandinavian as I feel like its easy to play for white even if he doesnt know the theory. Just develop the pieces and white is fine I guess :-)
For the carokan I do agree that white can "drive" the pawn structure and that it can get tricky...
The Benko looks interesting.
What about another Sicilian? With e6 and a6?

They are very different . First , Carokann requires More Theory . 2nd , at carokann , white Will choose How The position Will be . If white is a good Player in closed positions , they can go with 3.e5 , If white is a good Player in open positions , they can go for Panov Attack . Finally , If white prefer semi - open positions , they can go for Nc3. Even though both goes to equality and 1..c6 is completely sound opening , as I see it is generally hard to be ready and play 3 different type of positions .And The "Same structure " only happens in one variation .
He did ask for a Main Line opening ... There's no coincidence that the Scandinavian is a fairly rare bird in top level chess, while the Caro-Kann has been a regular feature in World Championship matches. Sure, there are more structures to master in the Caro-Kann, but the most common one to encounter is the one you have in the Scandinavian, but without making two queen moves inside three moves. The other structures are well-known: the Panov-Botvinnik attack occurs also in the Nimzo-Indian and the Queen's Gambit, and the Fantasy and Advance Variations are basically the French (I'd even say that they're more comfortable iterations of the original). No reason not to try to master them for blackpopa, especially since he's hoping to improve.
On d4, I play the KID but I feel like its good on blitz but tough in longer games... I also play the Benoni but feel like I am way too weak to play it well :-) And a single bad move in the Benoni gets you crushed very quickly :-)
What would you guys advice?
Cheers