There's a 2004 game by Platine Dispositif (devs of Bunny Must Die & a bunch of shmups) called Dicing Knight. It's a action-roguelite with bullet hell elements, and apparently even has an extra character whose main attack is shooting rather than melee. It predates the action-roguelite-multidirectional-shmup-hybrid boom by pretty much a decade, unless I'm missing something like it that came out before.
When talking about shmups, I often make a joke about how just when you think you've gotten an original idea, you'll find a 2000's doujin game that did what you're thinking of ages ago, and probably did it better. The doujin rabbit hole is quite deep and there are a lot of really interesting cool games to discover like the Sin & Punishment inspired cabal shooter Iwanaga, a bullet hell autoscrolling run n gun Lilian Fourhand, a whole bunch of awesome shmups like Hellsinker, Suguri/Sora, French Bread's early games, batshit Gauntlet successors, etc. Even the shmup community is rediscovering a lot of this stuff, like the Tsukihime shmups. And of course there's a whole world of doujin fighting games that I've only casually explored, one which Melty Blood came out of. You can browse through Zaarock's youtube channel to find a bunch of doujin games he's played over the years.
What's striking about these games is the quality - these games look pretty good, they sound pretty good (or excellent even), they feel incredible and they are not just well designed on a fundamental level but often quite innovative, as Dicing Knight demonstrates. I think there was something special going on at the time, and it was mirrored by the big boy devs on the PS2. Something that lead to this crazy, unique, innovative but fundamentally sound wave of games.
I'm a big proponent of understanding the conventions of arcade genres, and some people are wary of this because they assume I want every game to be the same. And sometimes I also wonder - what's the goal? I think this era of games, both doujin and more mainstream console games gives a really clear answer. The goal is to internalize conventions and then transcend them. Freely being able to bend and mix and match anything while still having that solid foundation of arcade game design holding it all together. That's what these games were.
They are games made by developers who grew up playing a shit ton of arcade games, who FELT them, who soaked in the juices of the arcades. They were then given freedom - either the freedom from the strict arcade model, the freedom from tech limitations, or just independence which allowed them to mix and match anything they wanted with no oversight. As a result you got all these games like DMC, Shinobi, Ninja Gaiden but also all these really cool inventive doujin games.
And that should be the goal, I think. To reverse engineer and internalize all this stuff, and hopefully tap into that same energy that I think made for one of the coolest periods in gaming. It can't happen without an intimate familiarity with arcade conventions, but it can't happen without fresh innovative ideas and without bringing said arcade influence into other stuff either.
