Shamelessly stolen from Resetera user Spring-Loaded, because it’s fun to rag on Nintendo...
Though it's difficult to directly compare different series because of different number of releases or time frames, and then try to gauge how well they were treated relative to what they "deserved," but let's do it anyway.
You can pick a series like Ice Climbers or Mach Rider because they've only gotten one game, or you can pick a series like Yoshi or Kirby where it's gotten many games, but you consider it's had too little expansion or innovation.
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I choose Star Fox, because despite getting a decent number of regular releases over the past ~30 years, the majority of those have been weird experiments that didn't necessarily serve the experience of playing them. The original and 64/Lylat Wars were great and well-received, but after that was a series of titles diverging away from what made the first couple games enjoyable.
Star Fox: Adventures (2002) came out 5 years after SF64/LW, and was a reskinning of N64's Dinosaur Planet, a Zelda-like experience. It was an OK game with some shortcomings, and a weird direction for the Star Fox series. It shared virtually no connective tissue with the previous game—even ignoring the obvious structural and gameplay differences, it wasn't even combat- or score attack-focused. It didn't capitalise on existing settings or characters, and, logically, it felt like a completely different series with Fox McCloud shoved in there. Anyone hoping for a better and/or more expanded vehicle-based shooter would not find that here.
Star Fox: Assault (2005) would go back to high-tech, fantastical warfare while introducing greatly expanded infantry combat comapared to the unlockable bazooka option in the N64 multiplayer. Having levels with fights and objectives taking place both on the ground and into the skies, or inside and outside a space station was a cool concept, but mid-2000s TPS controls did not make for an especially polished experience. Entering and ejecting out of ships seamlessly was cool, and all the weapons and vehicles made for potentially fun multiplayer despite the controls. However, the main game's short campaign, its less briskly-paced, replayable mission deisgn compared to 64, and lack of branching level paths felt like set-backs that the novelty of on-foot, vehicle-swapping mechanics didn't make up for, for many.
Star Fox: Command (2006) brought back branching mission paths from 64, and included multiple playable characters in its campaign, each with unique ships with their own abilities. The branching paths would lead to different team compositions, affecting the plot and character interactions, culminating in 16 different endings. The missions involved plotting courses through maps, drawing paths for each wingman to take, and trying to intercept enemies and missiles targeting the player's mobile base. While all that was cool, the samey gameplay sequences that were almost entirely "all-range mode," made for a repetitive experience throughout—just generally open, flat and/or empty environments. Enemy variety could only go so far in combatting the uninteresting structure of encounters.
There was a Star Fox 64 re-release on 3DS (2011), and it was cool. Not much done with it beyond visual upgrades though
Star Fox: Zero (2016) brought back a focus on score attack gameplay, on-rails levels, and vehicle combat. However, it was built entirely upon the dual-screen concept from a Miyamoto prototype. The control scheme did function, and provided a potentially higher-degree of control for players where they could aim in a different direction while steering the ship in another ... but this did not prove intuitive for many, even hardcore fans of the series. While the gyro aiming allowed for more challenging all-range mode encounters with more mobile enemies, they also served as a barrier of entry that wasn't worth it to surmount for subsets of the audience. The focus on building a game around this concept and having it render two separate views simultaneously meant the visuals could be underwhelming in spots, and while there were a few standout levels, there were some that clearly felt tacked on and under baked, likely added after it was delayed from 2015 to 2016. A divisive game, and one that did few favours for the series, which is now in an uncertain position.
The "cancellation" of Star Fox 2 (2017) also sucked, and while it's neat it got to release 22 years later, it was denied a chance back in '95, even though Super Famicom titles would continue to release past that time. Even if it looked bad relative to PlayStation output, graphically, it's still a bit of a missed opportunity for the series.
I nominate Star Fox because it has almost never gotten a chance to get an improved sequel—each game feels like some weird, new experiment rather than a game fundamentally built to be an enjoyable experience. For the level of mindshare the series has (typing "do a barrel roll" into Google still does the thing), it has been handled messily.
I think when we’re talking about ‘mishandled’ as opposed to flat out dropped (Sorry F-Zero), it has to be Star Fox doesn’t it?
I mean Paper Mario probably hurts a bit more because of how much I used to enjoy it but it really seems like Star Fox could be something if the higher ups weren’t so stubborn about how it was rehashed. It has a fun cast of marketable characters but man they just keep churning out gooseberry fool when what the series needs is so obvious to everyone else.
Metroid I think would be a close second, again because of the mass market appeal the series could so obviously have. At least it kind of seems like they’re trying to give it a bit of a reboot though. We’ll see.
Interested in what the rest of GR thinks.