Don't bring finisher mori to live
I agree that the yellow mori should be made basekit and that in animation it should be clear and not have debris and map elements blocking the view like having the survivor's head through a wall or door. Unfortunately that is about the only parts left I agree with.
Killers bring mori's for various reasons whether they think of it as a fancier more interesting way to get a kill, they're doing a challenge, they're doing a daily, they suspect people in lobby are playing together and may be more altruistic for saves, etc. Survivors for the most part nowadays do not mind mori's, in the case of some like Ghostface they even find them entertaining and like to be a part of it while the killer is locked in animation. This part cannot happen if moris are restricted to only the last person. At this point the only moris that still have some annoyance and resentment attached to them with the community is the stare from condemn only Onryo players that would not hook and would only teleport around until finally someone got fully comdemned or the tombstone addons for Myers. If this change goes through these would be some of the only options left available if a killer wanted to mori, which increases the distaste in people for the element of gameplay. People do not mind being mori'd fairly when they are on death hook, they don't enjoy possibly being killed before even taking a hook state.
When it comes to doing challenges or dailies the kill someone by your own hand is already one of the trickier ones, but it will be even harder if you have to have only one person alive. Survivors do not enjoy being left in the dying state in general but they especially dont like it if the killer has no idea where their teammate actually is and leaves them there for multiple minutes trying to find them. This already happens occassionally with players that want to kill everyone, it will happen more if people have challenges tied to moris where they will have to try again and again if they take a risk and the last survivor finds hatch. The only killers that have moris into their basekit come from licenses and thus cost real money to buy, and even then they are not a 100% guarantee. Condemned kills from Sadako are very rare unless you dedicate your entire playstyle around it and do not chase or hook, which again is not very fun for either side. Many survivors dislike the tombstone addons and will do everything they can to deny being mori'd including locker standoffs that can go on for over 30 minutes. Vecna's built in mori can only be activated if someone in the match rolls a natural 20 and acquires a special in-game item so it's possible to play multiple games and have no one get the item.
As for the perks that allow you to mori multiple times Devour Hope is a hex that has always been inconsistent either providing amazing value and winning a game or being found relatively fast and having no impact. Rancor also is not consistent in that most experienced players will notice it and stealth in the endgame and not participate in any saves or even leave immediately. That is why I propose that we leave the ivory and ebony offerings to function the same in letting you mori outside of the endgame, maybe change the ivory where you can do it to 2 people since yellow would already be basekit. People want to use these offerings but you will not see them used if they are only extra bloodpoints, people will simply let them rot in their inventory and play extra dirty (also we haven't even gone into the people that bought legendary skins with unique moris!).