WE ARE IN SERIOUS NEED OF A COUPLE MORE ISLANDS.
There are several different formats incress the # Of Habitats. Maximizing space with these formats, space is an issue.
Here are (3) ideas to fix this problem his could potential help.
(1) INCRESE the number of Monsters each can hold. With this it would only be temp. And concerns of the upgrade cost could be an issue especially with (Legendary)
(2) Another idea would be to RETIRE some of the lesser used weaker monsters.
(3) ISLAND Add Another Island.
THIS IS BECOMING A BIG PROBLEM EVEN WITH THE MAX NUMBER OF MONSTERS ON EACH.

Thank You

Sp addressed this many times already and the final answer is no more islands

1- they said most devices are medium-tire one and more island or raise habitat max will increase the load on those device
2- The vault . Sp think it solved the problem bu introducing it and want you to use it

@r_vale1961 Retiring the less used monster (i.e. commons and uncommons) is actually a good idea I haven't heard before.

Even some epics can retire IMO. Who actually uses Rhynex and Erpham? The game is about buying more and more powerful monsters (legends). There is no ePIC mETAGAME, in which case there isn't a Rare Metagame or an Uncommon/Common metagame either.