There's so much that you can do with these skins Riot and you fail. Pentakill Mord? Why can't it be a concert when he ults? It's literally the same skin as infernal.
I'm not sure if you realize, but Pentakill Mordekaiser is a 975RP skin. That tier is mostly just VFX changes, I.E, fiery FX instead of ghostly stuff. I'm pretty sure it'd need to have it's price bumped up to 1350RP if not higher if they were going to add a bigass concert to his Realm of Death, considering the huge amount of work it'd be for...Y'know. A 975 skin.
I also feel like every rework skin in the game has 1 skin that focuses on blue and 1 being a fire theme. Can we get more effort?
Can you put more effort into your description? Fire, in general, is an appealing thematic, and Riot clearly has an easy time spinning off interesting skins from something as broad and general as fire.
And, something I think you're ignoring is the fact that they're updating pre-existing skins, not making new skins entirely with a VGU. Since I'm going to assume your irritation with "blue themed skins" is referring to Full Metal Pantheon, I want you to look at Full Metal Jayce. A skin that shares the name, but looked vastly different to the jagged-metal amalgamation that was Pantheon, and, most notably; had blue.
Very rarely will Riot have to turn skins into something new with a VGU. They had to do it with Infiltrator Irelia (no outstanding thematic difference from Nightblade), they had to do it with Crimson Akali (what good is a skin that's just a recolor when chromas exist?), and, with Pantheon, they had to deal with two separate skins which lacked a unique thematic difference from the base Pantheon, and one skin which was comically outdated compared to another skin which shared the same name.
The former two being Myrmidon and Ruthless, and the latter being Full Metal.
And you have to admit, it had to have taken them a fair bit of effort to land on the designs they chose to run with.
Lord Mord just has a purple passive... why can't you add rain and more water and purple effects? Look at his amazing splash art.
Having rain in a splashart doesn't mean that his VFX should focus on rain. Regardless, post-VGU Lord Mordekaiser is supposed to be something like an Eldritch parody of a Cleric/Paladin, and purple/green is a pretty standard color palette for eldritch magics. Green especially, considering that Lord Mordekaiser (with his tentacle horns and pauldrons invoking the visage of Cthulhu) has this imagery of "terrors from the deep".
But Perseus could be so much better. Can he have water effects? And cast a wave when he lands for his ult? Also after he blocks the damage could you add a medusa scream? His shield is Medusa.
Much like it's worth noting that Pentakill Mordekaiser is a 975RP skin, it's worth pointing out that Perseus Pantheon is 750RP. Skins of that tier typically don't get anything more than a change to physical appearance; all VFX/SFX stay the same.