Ya know, I'mma play devils advocate for a second and say that GtI wasn't actually that bad when considered on its own. The main things wrong were the length, which is a moot point when considering the literally endless nature of the procedurally generated dungeons and maybe the story. Looking at it as a stand alone, it was pretty decent all in all. It kept up with the music imo and the core gameplay was as good as ever, aside from the weird little over-worlds we had to explore.
Thing is, everyone prefers the prior games and the drop in quality was too much to ignore. They took out the personality test, which was a lovely little touch that made your own mon feel personalised to you and replaced it with a really limited and straight forward choice. the 3D models were too few to have the same impact as 6th gen and cost the game the charm we loved in the pixel art and sprites and when we do actually compare the stories, GtI doesn't stand up to either of the Explorers.
So, what can super do to improve? Personality quiz for one, more starter variety, earlier team customisation has been a recurring issue in the series which PMU went ahead and solved on its own. sticking to the 2D guns would be nice, improved dungeon generation and then the main challenge will be holding up a half decent story.