Since I'm really stanning these kinds of bands right now, I thought I'd make a topic for what I like to call "transition-kei". Basically, a made up term for those bands in the early 2000's that were in between old school and modern.
Personally, my favourite at the moment is Cuartet. They were like The GazettE (which Saki is part of a proto-band of) and Gill'e Cadith, but much heavier. They also had more Nu-Metal influences. The combining of the old school Thrash Metal influences with Nu Metal in a way, sometimes resulted in groove metal-like songs, often reminding me of American bands like Machinehead (though IDK whether they have actual Machinehead influence). Saki wasn't the best vocalist, but he knew how to work with what little he had, and didn't try anything outside of his ability for the most part (unlike some of the vocalists from that era...)
Another favourite of mine is Gill'e Cadith. I briefly introduced them to @suji, @CAT5 and @plastic_rainbow on New Years Eve on Plug.dj, and I've been meaning to go more in depth here on the forum to give more insight (even though there are much more qualified people to do so. Rarezhut's YT channel has a really good video on them). In short, Gill'e Cadith was this really quirky band formed in 2000, out of the ashes of the already popular (in the indie scene) Cradle. They had really odd sounding music, and was sort of a split from the post-punk influenced music that defined the 90's. The vocalist, Karuna, was realtively new to the scene at the time, but fans of Deshabillz and Canary saw some familiar faces, as their bassist also played in Canary, and was in Deshabillz and one of their ex guitarists was also in Canary.
They were also somewhat influential, as their quirky style caught on with other bands pretty early, such as The GazettE and Cuartet, especially Karuna's signature glasses-look. (though I'm not sure if he started it). Basically, they were one of these once really popular, but now forgotten bands.
Karuna to this day is still active, as one of the vocalists for the band 君は鋭く, and was also the first vocalist for the super-group Number Mouse, for the really short era where they were Vkei.
However, for more in depth, I do recommend you check out Rarezhut's video, and Shattered Tranquility's article on them.
Anyways, anyone else got some good transition bands they want to share?