So, if this customization were to be enabled, I think it would become multi-mod supportive, and would quickly gain attraction through mod players on YouTube, Twitch, etc.
Another way of phrasing the suggestion is to enable transparent parts of selected entity models to become see-through portals. Lots of mods, such as the Dalek Mod, Dimensional Doors, and many others use transparent parts of entities as a hitbox to transport the player to another dimension, although not very seamlessly. If you could integrate the immersiveness to
model-specific transparent textures, that would make mod playthroughs so much better.

Having dimensional doors with see through portals might be difficult. Because multiple doors can lead to one. For example, in a quartz dimension you can have multiple exits that lead to the same door, and also having multiple doors at the overworld that lead to the same door at the quartz dimension. It's very tricky. It won't be coherent with other entities.
For example, 2 doors at the overworld A and B that lead to the same quartz door C. If one entity goes through A to C, looking from B it would appear out of nowhere. And if it goes back again from C to A, looking from B you would see it disappear.
This could be fixed by only allowing one to see through the door. It would be amazing to have dimensional doors with see through portals, and it would be a nice feature to quickly implement other mods