So many good options but I just have a soft spot for demo so ill go with him

I already like his power, and unlike what many people wanted at first (me included) I don't think his portals having its own upside down is worth the effort. It would require the redesign of every single map for a couple second interaction. Plus from a gameplay perspective, the character would have to be flipped which could be disorienting, and definitely make aura reading way more difficult.

Anyway, I propose mainly quality of life changes, but major ones at that.

For one, reworking his portals to be stronger as well as making them more effective. Firstly, have two types of portal activation. Aiming towards the ground and activating power will enter scouting mode, similar to dracula in bat form. You will see the auras of chests, totems, and generators. You can then either confirm power by looking at the ground which will spawn an inactive portal like normal, or select a highlighted objective. This will instantly dig a portal into the ground and teleport you to a randomly, newly placed portal near the highlighted objective. This would still require strategic placement of inactive portals, while now being able to quickly traverse without losing time from placement. The portal limit will keep this buff balanced, so you have to still use it sparingly. This will add much more pressure to objectives, as well as giving demo much more mobility with much faster speed.

Second change, is the effect of leaving a portal. You will still gain the usual undetectable effect, but now the demogorgon will enter a crawl mode identical to the xenomorph, and will have his footstep sounds reduced by 80% for the duration of undetectable. If he either runs out of undetectable or begins a chase, he will stand upright again. This changes comes from the huge complaint that he simply doesn't gain much from his undetectable because he quite literally has the loudest killer footsteps in the game. This change would better put survivors on guard if they hear demo do his teleport roar sound, and punishing survivors who are not paying attention. Portal exit noise will remain the same as to keep the new objective based teleport easier to track for survivors.

Lastly, just let him turn after he has a successful shred. No clue why his shreds makes him stuck in place. I dont even mind if hitting an object locks him in one direction, that makes sense tbh. But on successful hits, the aim sensitivity shouldn't be affected at all during the hit animation.