@Egzample I understand you, I believe that the "this is a video game" concept will only be used to enhance the story, not be the big story bomb at the end. I'll explain: The voice in the Citae and at the end of Episode 18 tells you in so many ways that it hopes to "keep playing" with you forever. It also asks us if we've ever noticed how there are always Star Shards laying around when we need to heal, or that there is always a way out of every predicament we find ourselves in. On top of that, it holds us in the black space for a time before letting us go free.
As far as the healing items and ways out are concerned, you would normally just chalk it up to standard video game level design. Reborn goes a step further in calling it out, but in a way that incorporates it into the story in a realistic way. The same thing goes for the black screen transition at the end of an episode; the player's suspension of disbelief would briefly vanish as they realize "oh, I've hit the end of the game". But again, Reborn uses this as a moment to build on the story as the voice briefly traps us in the void.
One more fun fact: while in the desert with Taka, he mentions that Lin is narcoleptic, and will drop to the floor unconscious occasionally, and no one is able to wake her up. I've wondered if this is a reference to how the player can save the game and then turn it off for extended periods of time. That would also explain how she came back to life after Titania stabbed her; just like how the MC also "respawns" if they lose an important fight.
So in conclusion, I don't know how much more the fact that this is a game will be brought up, but I highly doubt that the story is going to take a hard left in favor of breaking the fourth wall.