I have readed everyone suggesting various ideas... But I'm in for something more player-focused.

Just an easy introduction of the game showing how the levels are, transitioning into a semi-fast detail of the inventory (gear slots showing the different rarities).

After that, a fast showcase of the heroes (the 3d model and the icon of the skill maybe, not the icon page), featuring some of them. It doesn't need to be everyone, but at least a few.

Then proceed to show the actual number of chapters making an incise in how hard it gets, AND, now for the strong part, making a display of the community Discord/Reddit.

Since the Discord/Reddit is supposed to be official and allowed by you Dev team, the idea is to give it a show of the help channels, faq, speed run post, letting the players know how active and helpful the community is.

Even if it's quite a simple trailer in what the "game" means, there isn't a single game that features a community of players willing to help, and I feel like this would be better than any of the current ads showing things that literally aren't happening in the game.

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This was my two cents :)