Technically, any guardian can use light/dark powers for anything. You could make a void gun, a solar staff, or an arc bow. Subclasses are simply just preffered forms of utilizing the light that is common among guardians (also gameplay mechanics). One example is a lore book from season of the chosen. Shaw Han, Lord Saladin, and some redjacks were fighting the cabal in the cosmodrome. Shaw Han used solar light to empower the weapons of the redjacks, making them shoot more powerful solar bullets.

All lightbearers have access to all elements but certain users tend to have certain affinities. Equally the class system seems to just be a way to categorize certain traits and affinities. Before the vanguard, classes did not exist. I would somewhat trepidatiously draw the conclusion that personality dictates both class and elemental usage. We know from D1 that solar usage is rare for Titans and void usage is rare for Hunters. I don’t remember as much emphasis being put on the rarity of arc usage among Warlocks but I would assume it is also somewhat less common. Of course, you get those few who can easily move between the elements, even converting one energy into another.

A similar framework like that already exists in the game and in the lore. As well as Guardians being associated with new or unique abilities....as well as specializing in certain subclasses/ways of using the Light. Osiris was a Solar specialist Warlock. Ikora is a void specialist. In her lore she talks about how difficult it was for her to learn to use the Solar abilities because you need to cultivate an inner stillness to use it, that her emotional intensity made difficult. Whereas the that character trait made her a powerful Voidwalker, but sometimes too powerful. While Stormtrance allows you to walk between the two, so she's very capable of using it. But whenever you see her reach for her power on instinct It's always void. Cayde-6 and Ana Bray were Gunslingers. Hunter tend to be very pragmatic in their use of the Light. The Light is simply a tool to get a job done....and just using solar power to infuse your weapons directly seems to be the most straightforward way to do it But it seems to come at a price where Hunters seem to be the most corruptible of the classes. While Warlocks----who immerse themselves most deeply in it seem to the be the least corruptible. Cayde once mentioned that void Hunters were actually quite rare. Void seems to be the aspect of LIght that is closest to the Darkness. So he spoke of Void Hunters being at risk of losing themselves to the Void. So the Hunters that can walk that edge and stay there are few. Titans are in-between. Both in methods and temperment. They infusese themselves and their weapons with Light...but tend to see themseles as the weapon. Plus their almost Jedi-like devotion to protecting/defending others make them difficult to corrupt....but not as difficult as Warlocks. You basically have to drive a Warlock insane to do it. But you can't corrupt a Titan by twisting their desier to protect and serve (Dredgen Yor) Zavala is the most versatile of the Vanguard leaders. He is equally comfortable in using Arc and Void. He tends to draw on void when his goal is defensive....but will tend to draw on Arc when he goes on the offense. He's had an historical beef with the group of Titans who promote use of Solar Light...so that is probably the only reason why we don't see him use it. The only reason why we don't see this more in the game is due to PVP balance....and the poor game design of vanilla D2. Y1D2 was designed for "pvp balance". Which is why the Light subclasses were so rigidly limited in build variety....and why Bungie is having to take all of next year to break all that down, and allow for more variety of builds. But also introduce more clearly defined purposes and powers to the different elements.

We do it all the time actually. Guardians utilize light in varying ways to suit what the situation calls for. Our gameplay is kind of stagnant, but the methodology is still fun and interesting when you dive into the lore.

Osiris can make solar copies of himself, we've seen Ikora convert nova bomb in a void energy beam. There are neutral guardians who developed unique ways of using light, they live in a commune on the jovials

The Power of the Light and Dark comes from the Same Source. It's how it's used, determines what it becomes. Like the Force. In the Beginning, there was Only the Dark Side. The Light side doesn't exist. But both Jedi and Sith, both use the Darkside.