I honestly don't think an arcana would be able to cut it with this. As mentioned below, arcana's can't significantly impact the visual silhouette and readability of the hero. Cosmetics (good cosmetics, mind) work because they look like something the character could wear but still look like the same person. The visual aesthetic (wings, bright colors, probably different magic missile particles, etc) could make these two look like entirely different heroes.
I can only think of 1 time when there's been such a massive overhaul to an existing character, at least recently, and that was Skeleton King. They turned his change into a full fledged event, and that's what you've got here. Skywrath and Venge have probably the best lore interaction in the game (barring kunkka x tidehunter), and the reclamation of the nest of thorns would be a fantastic summer event.
Picture it like this: a new update hits the blog, showing the massive skywrath realm, perched above the clouds. Skywrath and venge are removed "for pressing ceremonial reasons" and we get the hint of a new game mode. Suddenly we've got a new queue called "reclamation", a 5v5 attack defense mode calling back to the old days of dota, with radiant vs dire. You can queue as either side, and will fight to either reclaim the nest of thorns for venge or defend it for the Skywrath queen-both with the promise of loot. But venge has to win, so it'll probably just be we play both sides for different sets of items and rewards, maybe different mechanics (tower defense? item selection?). Once the event is over venge gets a brand new model and lines, skywrath gets some new lines and maybe an arcana himself (Favors of the Queen Reborn set?) and everyone's got a few more cosmetics, some fun memories and a new game mode to bitch about.