Some of these suggestions go against the purpose of the monster vault so may not even be considered. The monster vault is fine up to a certain point but the cost, even with a discount of unlocking slots starts to get prohibitive.
I am sure many would like to keep some monsters in some form (egg/cells) in case it is required in the future (breeding event etc.) but it may not have a value being hatched and strutting around a habitat. I know very early on in the game I hatched everything I had as a means of gold production, but they are of little value now.
I would like to see more options in the game for what we can do with the last instance of a monster. Currently the only options we have is to sell the monster, no option to extract to cells. A couple of things that would be good:
Extract monster to cells with adequate warnings this is the last instance of the monster.
If number 1 is going to cause too many support tickets because people didn't read the warnings then the option to extract the monster to cells only if you also have that monster in storage.
Revert monster to egg and place in storage. If monster is ranked then excess cells above Rank 0 added to your cell vault, total cells extracted minus the equivalent of monster fully fed at rank 0.