I always lament that The Guardian Legend by Compile didn't have more influence on the SHUMP genre.
TGL was a merger of shooter and Zelda; a SHMUPtroidvania, sorta.
It was a hardcore shoot-em-up, but it also had RPG aspects in an overworld to give you a variety of options to take if you're getting your ass handed to you. You can upgrade your character (but with limits on the EXP system so you don't just grind your way to overpowering stages... you still have to git gud,) you gain a huge variety of unique weapons (which have shared ammo resources, so there's a give-and-take between the awesome ones and the ones that drain less resources,) you have on-foot sections which uses similar weapon techniques but play differently (and sometimes have puzzles to them or reasons to come back through them,) and you often have access to multiple SHMUP stages at a time to push different skills or earn new abilities if one stage proves too tough. The game's extensive run and multiple collectibles link with a password system to maintain your progress. The SHMUP stages can be brutal (the opening alone is an assault on your senses with a high-speed trench run where obstacles come almost faster than you can see them, followed by a boss that is just a massive wall of laser-shooting eyeballs... many don't make it past the first 3 minutes of gameplay,) and its enemy system is rather unique in that attacks often home in on you instead of littering the battlefield, so you play with an eye on your energy and clearance power rather than trying to be perfect, instead using the full range of your health bar and using risk-reward for what power-ups you chase when they appear. It's hard, it's varied, and it's what I thought for decades would have been the "future of SHMUPs", but it never happened.
Over the years, there have been a few attempts at doing something like The Guardian Legend, but not many. Most notably, Wayforward had a well-intentioned, ill-fated GBA game called
Sigma Star Saga in its rising-star period as a little but notable GBA developer (it was published by Namco, but I don't think it had enough funding/time for the project they were trying to make and also SHMUPs take certain skills that maybe Wayforward didn't have... despite returning to many of the gameplay systems it explored in the early-aughts, they've never tried a spiritual successor to Sigma Star Saga or fit SHMUP gameplay into any of its other licensed/indie projects.) Before then, there were a few games that combined flying and exploring, but not quite with the same formula.
Akuu Senki Raijin on FDS has both flying and shooting, but they're on the same level maps in a weird hybrid style.
Compati Hero Great Battle Gaiden 2 on Super Famicom had SHMUP stages as one of the minigames you could jump into from the central platforming levels, but these are just among the multiple minigame types (including an Arkanoid minigame and other play types.) And then, a few indies have over the years tried to do their own TGL, but most never made it through development.
Steel Seraph came out in December and is ugly but inexpensive so maybe try that on PC. There's also a beta out for the seemingly abandoned
Zodiac: The Guardian Story. And there's an interesting project called
Astrolancer which has been in the works for a while but doesn't have a finishing line set yet and it's also more bullethell than typical SHMUP. Also, some home developers and indies have tried making HD versions of TGL, but they've all crashed and burned as far as I know.
Still could be the future of SHUMPs (or at least "a future", for developers avoiding the genre because it doesn't have the epic, lengthy feel needed to attract sales outside of SHMUP fans) if only some developer would explore it...