I’ll answer some of your questions here.
1. What actually is in that kind of games, so you spend so much time?
For me FPS games are my bread and butter. I used to play a ton of games when I was younger, but these days I play maybe 1 or 2 games these days. Right now the only two games I’m playing are Battlefield V and Assasins Creed Origins. Both offer unique challenges and rewards.
2. What is annoying in that kind of games?
Multiplayer games always bring in some toxic players. Thankfully it’s not as big of a deal in Battlefield games, least from my perspective. I have been accused of hacking and kicked from matches in games like Counter Strike because some players didn’t like losing which is annoying.
3. What are you willing to pay for (new vehicles, tuning, )?
The only time I’m willing to pay extra beyond the cost of the game is for things like DLC. The only time I bought DLC is for the Battlefield 1 maps, and it was well worth it. New weapons, vehicles, maps and uniforms was a great thing to add in. I don’t mind paying for DLC as long as it expands the longevity of a game. I don’t pay for cosmetic items.
4. If you have got any ideas please share them here.
For your specific game you could have new vehicles or more maps, but it depends how deep the game would be. It seems to be a mobile game that most people would play here and there when they are bored on the train or something like that. I would instead focus on creating a more addictive core game and sell that directly.