"There were obviously concerns from the art team about how to make this not look stupid in game..."

In the Champion Insights: Taliyah article, it was mentioned that an early ult idea was for her to be able to destroy walls, one example being the wall around Baron Pit. It was scrapped for being more of a support ability, but as quoted in the title, the art team didn't know how to make it look good in game. However, after thinking about it for a little bit, the answer seemed rather easy: banish it to the Shadow Realm.

Namely: this. Specifically, the texture on Kassadin's void blade thingy. You can tell the blade itself has shape to it, but looking directly at the texture just makes it look like you're looking through a portal into the void. Take this, then, and stretch it out much bigger and you have your answer.

You have a support character that can place a zone on the map. Everything within this zone, including walls, is covered in the same or similar texture and is temporarily made an area where movement is allowed (as opposed to currently where the walls don't actually exist, they're just the art asset that sits on top of an area where movement is not allowed). Nothing needs to be done to the art of the Rift itself, or any map for that matter, and it looks fine because it's this massive magical energy field that is sitting on top of the map.