MM8BDM is fun to map in but however, one feature that would be really appreciated would be linedef portals
Why? Linedef portals bring alot of possibilities to mapping, such as making non euclidean environments, looping areas, more CTF map ideas, etc. While yes we can technically create portals by using a script but it is notably off and can be hard to set up, so i think linedef portals are a must have.

Is there a point to having a custom tool for it? I don't imagine a lot of maps will be using it aside from a couple for aesthetic/gimmick purposes? And also no maps for the vanilla game have anything of that sort, so It seems less likely for us to have a simple tool/object for it unlike jump pads or water, which are used very commonly throughout the vanilla maps. I mean I'm no vanilla dev and ultimately the decision is up to them, but from what I can tell having an easier way to make them doesn't seem to be a priority.

There is somewhat a point, even if small, although it wont be used in vanilla at all it could be used for minor details, and give more mappers a chance at creativity, although i agree it isnt much of a priority.

This isn't up to MM8BDM either way, this would need to be implemented in Zandronum.