Love the concept, and a game which focuses on customization of mechs and rearranging parts into creative new configurations sounded really cool to me!
The document did a good job of explaining the overview of the game and the main progression/upgrade pathways. I would have liked to see more detail on the specifics of combat mechanics and how rearranging parts can help with that. There were some intriguing descriptions of mobility-based parts like legs or hover-jets, but not combat-focused parts like gun types - which felt odd because high-octane gun combat seemed to be the main focus of the gameplay more than mobility or exploration. More specifics of how exactly you would combine parts to create different mechs - how the interface would work, what are the restrictions, etc - might have been more useful than the menus and save system description which is less unique to this particular game.
Likewise it would be nice to read more about how the different types of enemies function and look in-game - the story background for each race was super interesting to read, but I couldn’t imagine how they’d look or fight without more info.
The format and content was pretty clear and easy to read, although some diagrams might have helped when it comes to describing things like the HUD.
As for the genre, the Mecha-like genre described already seems to have a fair number of titles, and the document doesn’t specify which part of the design is genre-defining. The comparison of ‘more real-time and fast-paced building process than Crossout’ is intriguing, but the rest of the doc doesn’t mention real-time building and only seems to let you re-build your mech at certain key points. Likewise ‘an engineering take on Spore’ sounds instantly attractive to me, but is hard to substantiate without more description of the customization system itself, and what makes it deeper or more creative than other Mecha-like games.
That’s my feedback - definitely looking forward to see more of this if you plan to take it further!