This will cover both the single-player mode and the multiplayer-related stuff as well.
So for the multiplayer, they really missed opportunities with some of the emotes and gear. First, the emotes, they could've easily used the adjust feature to have some extra emotes. For example, one emote from Splatoon 1 has not returned, and they could've easily brought it back using the adjust feature. There were missed opportunities with the gear as well. None of the Splatoon 2-related singleplayer gear returned, which is strange considering shortly after Side Order's release, Splatoon 2-related content has gotten most of the attention.
Now for the single-player mode. The Return of the Mammalians did have some really big missed opportunities, firstly, it was going to feature all three of the previous games' protagonists in some way, but that obviously didn't happen. Another missed opportunity was having DJ Octavio replace Craig Cuttlefish as the mentor. The new Captain doesn't talk at all, so maybe we could've had DJ Octavio talk throughout the missions in addition to the Squid Sisters, we could've heard his side of the story, revealing a lot more than we knew before. This would've been very fitting considering Splatoon 3 is the end of the current story arc.
So for the multiplayer, they really missed opportunities with some of the emotes and gear. First, the emotes, they could've easily used the adjust feature to have some extra emotes. For example, one emote from Splatoon 1 has not returned, and they could've easily brought it back using the adjust feature. There were missed opportunities with the gear as well. None of the Splatoon 2-related singleplayer gear returned, which is strange considering shortly after Side Order's release, Splatoon 2-related content has gotten most of the attention.
Now for the single-player mode. The Return of the Mammalians did have some really big missed opportunities, firstly, it was going to feature all three of the previous games' protagonists in some way, but that obviously didn't happen. Another missed opportunity was having DJ Octavio replace Craig Cuttlefish as the mentor. The new Captain doesn't talk at all, so maybe we could've had DJ Octavio talk throughout the missions in addition to the Squid Sisters, we could've heard his side of the story, revealing a lot more than we knew before. This would've been very fitting considering Splatoon 3 is the end of the current story arc.