Which do you play, e4 or d4?
How would you start your games as white, e4 or d4? Statistically speaking, I'd assume the vast majority of you will say e4 as it is the most popular move. And maybe the reason for this is because complicated positions can be more fun. For those who are not a fan of tactics (like me!), maybe you play d4 instead because these positions very often lead to quieter and very positional ones.
But are you forgetting 1.Nf3? Don't deny it, I bet it didn't even cross your mind that people play this when I asked whether you play e4 or d4 (but congratulations if you did)! This opening is kind of a d4 opening in disguise because many variations lead to d4 opening positions, but unlike d4 it is very flexible and may reach many different positions!
Why Nf3?
1.Nf3 (or also known as the Reti) is full of numerous different lines, but almost every single one of these lines are playable! So even though it appears that you must learn a huge amount of theory, this is actually not the case if you won't get punished for playing the wrong move! But I will say that it is better to stick to a line then to keep switching it about every game.
Along with this, you can also increase the pyschological eval bar to +0.5 as your opponent can get really confused! Don't get me wrong: this stuff is not bad at all so you aren't sacrificing your position to do this, but it is so rare that when people see it they often don't know what to do!
One final thing is it is super positional-chess friendly for those trying to improve here. You can get numerous different imbalances which you can learn to play with and against from IQP to Queenside majority and may even enjoy a dominant space advantage in many lines. In some games, I even get to pull off positional exchange sacrifices!
So, why not give 1.Nf3 a try? If so, I recommend 1.Nf3, 2.g3, 3.Bg2 where you can fianchetto and try to use this long diagonal later. I personally feel that delaying d4 to maintain the flexibility or this opening suits it more, but then again it is up to the player!