Unfortunately we already know it's some sort of gun with bullets.
In Thailand each teaser came with a text.
Here is Garen's:
"That time when the sound of gunshot echo, it penetrates into the hearts of everyone who heard that sound. The sharpness of the bullet passed right through everything, not even an ultimate armor can stop it. As the crystal shines flashily, the only hope of Demacia is destroyed."
I wanted an assassin or a mage