Marching Band Skin Lineup
I'm not the best at art, so I don't have any drawings of the concepts for these skins.
Basically, marching band season is starting up, and I thought that some marching band skins would look really cool on some champions.
Here's the 4 champions I thought would benefit best from the skins. There may be more, but these seemed like the easiest to begin with.

BASE MARCHING BAND SKIN
The base skin for each of these champions would have to be a marching band uniform that's themed after the riot logo, with the basic color being red with white highlights and the Riot logo on the chest. The pants would be red with a white stripe going down the side. Essentially this but slightly different. Hopefully a red marching band hat and plume could be added as well.
The four champions I chose were Jinx, Tristana, Sona, and Gragas. Mostly because they fall into similar stereotypes that my band has, where the brass player is loud and boisterous, the woodwind player is friendly but has some attitude, the percussionist is a headstrong jokester, and the conductor is quiet but gets things done.
Marching Band Jinx/Brass Player Jinx

Jinx being obsessed with guns reminded me of a lot of my band members being obsessed with instruments, where most of them not only take care of their instruments like they're human beings, but also have way too many of them.
Fishbones could be a Sousaphone, firing a Bass Clef note
Pow-Pow could be a trombone, Firing Treble Clef notes
Her Zap! Could be a trumpet, that fires out little music notes
Her Flame Chompers could be flip books that flap open and closed
Her ult could either be a brass mouth piece or just her Ult painted red with something music related on it
Her dance could be her marking time or marching in place
Jinx isn't really all the music related, but because all the brass instruments are so similar, but yet so different, I thought that Jinx would be a great way to represent that, due to Fishbones being big loud and explosive whereas Pow-Pow is smaller, is less explosive, but it more accurate and articulate.
Marching Band Tristana/Woodwind Tristana/Clarinet Tristana

At first I thought that Tristana wouldn't be a wise choice, but then I realized that she not only sings during her dance, but Boomer looks like a clarinet!
Boomer's texture would be similar to a clarinet, probably looking similar to Squidward's than an actual clarinet.
Her auto attacks/abilities would shoot either wadded up pieces of paper, or clarinet reeds
When her explosive charge detonates, there could either be an explosion of music notes or tiny wood chippings from the reed that scatter about
Her buster shot could make a squeaking clarinet sound, and could be a larger ball of sheet music or a wavy looking note to show dissonance
Tristana's dance is already her marching, so maybe some new voice lines for that dance would be viable.
Marching Band Gragas/Percussionist Gragas/Base Drum Gragas

Gragas took the longest to figure out, because usually you would have to wear the drum with a harness on your shoulders, but then I thought "If Gragas can carry a full barrel of brew, couldn't he also carry and play a bass drum like he does his barrel?"
The bass drum Gragas is holding would have to be a similar size to his barrel
His Barrel Roll can make a timpani slide noise from a low pitch to a high pitch when it explodes (Bwooing)
His Drunken Rage could just be him banging on his drum for the 1 second that he would consume his brew
His Body slam would just be him lunging forward
His Explosive cask would be his drum either breaking, being pounded on, or just exploding
I imagine his dance being something along the lines of him setting his drum down and playing a few beats of a drum solo
Marching Band Sona/Conductor Sona/Drum Major Sona

Sona would be the one leading the band, due to the fact that she seems the most musically inclined.
Her Etwahl is a conductor's stand, or three regular music stands with sheet music on it
The sheet music is sent out for her basic attacks
Her Hymn of Valor could be a Conductor's Baton being thrown at a champion
Her Aria of Perseverance could be Sona conducting a standard 4/4 pattern
Her Song of Celerity could be either her conducting faster, or a metronome clicking near her at a faster rate
Her Ult could be her conducting a cut off for the other team
Something really interesting that the game developers could do is that instead of the usual dance for the champions, they actually play a few notes of a song. And, if two of the champs with the skin are together, they either play a different song together, or they play the same song but with more parts and it goes on longer. Basically when the 4 champs would be together, there should be enough instruments to create a somewhat simplistic marching song with Sona conducting.
I hope you guys like the ideas! And hey, if this skin concept gets enough attention, maybe we'll see if some of the Riot employees are band nerds too.