This article takes a look at the core game design reason that Anthem became a critical failure. Kotaku published a very long article interviewing multiple people from Bioware that describe the 6 years of project management hell. The reoccurring theme is that no one on the team knew what the game was about.
This idea of understanding your core gameplay loop early is vital to the success of a game. Everything else is built on this main piece of gameplay. Games that figure this out early and then iterate on that one idea can be very successful. A recent example is Slay the Spire. Their core gameplay loop is battling enemies one floor at a time while you build your deck. The bosses, stores, and different gameplay modes all support this main loop.
Anthem, on the other hand, appeared to know that they were a 3rd person shooter, but lacked the knowledge of how to close the loop. Would there be levels, raids, or is it completely open world? Even the movement was unknown because flying was put in and taken out multiple times throughout the 6 year process.
The takeaway is to figure out your core gameplay loop as early as possible. In other words, find the fun!