I do not know the others but I am tired of always being with the same elements as always, why not add new elements?
What I propose would be to put four equal monsters at the maximum level, an element can be improved, for example four firesaurs to the maximum of any rank (all monsters should be of the same rank and that they are at the maximum)
you can create the element "Pyromania" and so on with the commons and maybe put another book, but you could only do this on the commons (Firesaur, Treezard, Rockilla, Thunder eagle, Mersnake, Tyrannoking, Genie, Light Spirit and Metalsaur) ,
and that this could be done in a new structure, or maybe add something to the library, the laboratory already has many functions,
and the improvements of the commons (or whatever they want to call it when they implement it to the game) would be in conlusion: merge four commons of the same rank, the more rank they have the stronger and better they will be, implement a new structure or improve one for example the library, these are like overloaded or something like that (that's the developers' choice), but for it not to be so good they would have to reach a specific level of experience to be able to unlock this structure or this implementation, put a new book that would be for these new monsters,
and that maybe a habitat and an exclusive temple for these new monsters.