I think the best option of integrating a new GUI framework would be https://github.com/tobspr/LUI . It was quite literally designed with the intention of being integrated into Panda3D and is a pretty good replacement compared to what we have currently. Hope to get some good discussion going.
For reference, a previous discussion on the topic: Developers: Favorite GUI framework?
 As for my own feelings:
A replacement for DirectGUI doesn’t seem like a bad idea to me, as long as it improves on what we have. I’m not familiar with LUI myself, and so don’t have feelings specific to it, but I’m interested to hear about where it stands right now.
You are not the first to post about that, and probably not the last. I begun to learn Panda3d 2 months ago and struggled a lot with directGui. I read A LOT of post about people complaining about it, and a lot of people with new features/updates or even entirely new Gui system. DirectGui has never been reworked and most of the contributions are lost in the forum, and I don’t know why. Forget the idea to enhance or replace directGui, it will not append, don’t lose your time. Code your own gui system or reuse some contributions to make directGui a bit usable.
Seriously, i used to code with the early version of gtk and qt 20 years ago, and they were better than dgui today. I would not use such harsh words normally, but dgui gave me to much headache.
From my general impression, I suspect that the devs wouldn’t be opposed to a new GUI system. However, I also suspect that so major a change to the engine’s functionality would only likely be introduced in a major version, perhaps even Panda 2.0.
The other issue, of course, is the decision of what GUI system to switch to–the thread above didn’t really reach a consensus, I think.