1 - This is a very DnD approach which I normally would be in support of, if done well, this could be a good addition and encourages team work, having a variety of different styled rifts like this could be a strong edition and reminds me of something akin to solo leveling with Jinwoo entering different gates and entering a different world kind of vibe. If there was a section you were stuck on and had to leave it means creating additional activity in the NW trying to find team mates that make up for an area you lack in to help you in the rift. The puzzles and trials could make for some fun RP moments as well and it means no rift is the same with just a different boss. Varying levels of these rifts would be desired for all levels of RPers I hope.
2 - This is also a good idea, but I think it shines more as a unique mechanic for varying rifts and every rift. This feels like a very MMO style mechanic so having it be a mechanic of certain bosses rather than entire rifts could be a fun way to introduce this, if you had rifts which could potentially have a selection of bosses and not just a set one, then it could make having these mechanics come up quite fun and interesting.
3 - This one I don't have much of an opinion on, it feels faster just from reading it, but I don't recall experiencing the halloween rifts so I will refrain from giving an opinion.
4 - I think having this option is good for feedback and new ideas, having to put all the creative flow on yourself can be a burden, allowing the community to help and pitch their ideas can allow their creativity to be put to good use. I don't think this idea needs to take too long, I would say you have a good idea and base for rifts already and use this thread with ideas to make tweaks and changes here and there, or brand new ideas entirely.
Overall, I think a combination of 1 and 2 would be a good idea, mixing these two idea's could create some solid and fun rifts and create great interactions between RPers, but I do not think every rift should be like this, some rifts should, perhaps smaller scale rifts could go by current standards with a few changes so the rifts don't feel completely drawn out. Especially those with less time on their hands, it would feel sucky to spend a large portion of time getting and grinding through a rift to fail and then be locked out for a week. I am not against this, but not every single rift.
I didn't really experience rifts unfortunately, I did not have the power or understanding at the time to attempt one, but from what I understand they went down well. A variety of rewards available at rifts and potential drop options are always fun to see, having more unique reward options for difficult rifts would be fun