I don't get how making new sprites is tedious, I really don't. It takes some time, but I'll be satisfying to see it in action after all.
Spriting well takes an insane amount of time, work, and the skill to make it as great as it can be. Games with
paid staff don't put this much effort into drawing this many sprites. And you're really telling us to go back to putting Xtreme Sonic or UglyKnux-style placeholders to put in something as
barely intractable as a shuttle loop gimmick? The only way to make this any useful would be to make a full-on wallrunning gimmick, and not only would that require even MORE sprites to do, it'd require a whole engine overhaul because the Doom Engine just
does not work like that. Knuckles' wallclimb and Wildo's wallrun are more-or-less hacks with very specific purposes. And even if you did, then you would need to
redesign levels again to add wallrun or even shuttle loop segments. And if loops are all you want, those dont work in 3D space very well because you can just jump through them and get through it faster, unless you put a dumb wall or gap in it, which makes the loop feel artificial. A loop in 2D can be a brief speed obstacle, or a cool rollercoaster moment for a rolling segment, but in every loop you see in a 3D Sonic game, it wows once, but is super shallow and artifical due to the hoops the game has to go through to make it work. Wallrunning would not be worth it to implement, and shuttle loops even then so.
You have no idea how much you are asking for, and how little of worth it ultimately is. Plenty of other games can offer "real" loops if you want them, but this game lacks them for plenty of good reasons. The team focused on what actually mattered, and shuttle loops do not matter.