Dunsparce has always been a very odd pokemon. Just looking at it, its design is all over the place. Tiny little wings that don't get it more than a few inches off the ground, a drill for a tail, a serpentine body with no arms or legs, but the weirdest serpent you've ever seen considering how round it is, and fangs? It seems like its creators were trying to take it into nine different directions! Well, I say that's exactly what they were trying to do!
Let's look at some of the possible evolution ideas for Dunsparce, just the ones that naturally come to mind:
Flying type: It has wings, but in this state they are very small and not too useful. However, perhaps Dunsparce has an evolution in which these wings become large, beautiful wings like a bird, and Dunsparce becomes a master of the skies! His eyes even look a bit like flight goggles already!
Ground/Rock type: It uses its tail to burrow into the ground and makes elaborate nests underground, so Dunsparce very clearly has strong ties to the earth. What if these were only further enhanced in an evolution? Perhaps he gets a stronger drill tail, and more features used for tunneling?
Poison type: it is already a serpentine pokemon, albeit a very strange one, but most other snakes in pokemon are poison types, or at least have a fair share of poison type moves and abilities. Perhaps in an evolution, it becomes actually serpentine, growing long and thin, and maybe those pointy things on the bottom of its face become poisonous fangs or spikes?
Dragon type: Dunsparce could definitely pull off the dragon theme. He can really do everything, similar to how dragons are masters of all elements. Most dragons in pokemon have extremely diverse movepools, with moves from all sorts of types under their power. Dunsparce is the same, learning moves from 8 different types just from natural level up, and can learn moves from every type except bug, fairy, and fighting via TMs, but dragons aren't known for bug, fighting, or fairy type moves anyways. Also, when Drampa was first released, people theorized that he was an evolution of Dunsparce, so it's not even that hard to imagine Dunsparce as a dragon. Additionally, Dunsparce's design is based off a dragon serpent from Japanese mythology called the Tsuchinoko.
Now, looking at all of these types, there is one thing they all have in common: Eevee does not have an evolution for any of these types! As such, I propose that Dunsparce could possibly be a counterpart to Eevee, where every type that Eevee cannot comfortably fill, Dunsparce can specialize in. Between these two pokemon, they would have every type covered. As for the types I have not already mentioned, it would take a bit more tweaking on Dunsparce's existing features to make him fit the types, but I can see all of them happening. Here's a few ideas I have:
Bug type: Dunsparce is pretty small, and resembles an insect larvae somewhat. This could all be leading up into his metamorphosis into a beautiful insect! After all, bugs normally look completely different from their larvae stages to their final mature forms.
Steel type: That striped pattern on Dunsparce's back and belly could turn into steel plates, and Dunsparce could turn into a very defensive and tough creature. Perhaps the means for doing so would be similar for the transition Onix makes to Steelix, where continuous heat and pressure from being deep underground for so long has tempered his skin into something much sturdier and metallic.
Fighting type: Looking at Dunsparce's face, it sorta looks like he's wearing a luchador mask. Now, I'm not suggesting he grows arms and starts beating up other pokemon, but I would imagine him becoming a special fighting type, using moves like aura sphere and focus blast. If Pikachu can be a luchador, Dunsparce can too.
Ghost type: Dunsparce's eyes are always closed, so how do we even know its alive in the first place? Seriously though, maybe Dunsparce has an ethereal aspect to him that allows him to see despite never opening his eyes. The pokedex also mentions that it spends most of its time motionless under the ground. What else spends most of its time motionless under the ground? Corpses. Perhaps for a ghost type evolution, Dunsparce opens its eyes and becomes translucent and ethereal.
For further evidence that Dunsparce could have many possible evolutionary specializations, just look at its stats: 100/70/70/65/65/45. Its attack and defense stats are both evenly split between physical and special, and it also doesn't really favor either offense or bulk at this point. This provides a lot of room for it to be molded and shaped into whatever stats and niche would fit its type the best, leaving Dunsparce with no real niche and making it a very weak pokemon. Also, one ability that both Dunsparce and Eevee have in common is Run Away. This further drives the similarities between the two, as both are skiddish, weak pokemon that can't really fend for themselves yet, until they evolve and find a niche for their power.
Overall Dunsparce just has a ton of potential in many different directions, and all of those directions happen to be types that Eevee does not yet cover. It seems to have been designed as a perfect counterpart to Eevee, so even though it has not received any specialized evolutions yet, I don't think its impossible for it to become a secondary Eevee, to cover all the types that Eevee cannot. Dunsparce is too weird and unique to just be left behind and forgotten as a useless, weak pokemon. He deserves to flourish, and have every one of his design details be highlighted and expanded upon. Dunsparce was always destined for greatness, greatness that would rival even Eevee's popularity!
Hey thanks for reading all the way through my theory. I'd love to hear your thoughts on it! If you have anything to add, or you find a problem with any of my logic, please share! I appreciate feedback and responses of every type!