Firstly, I don't accept that playing a role in one studio's movies should prevent you from playing a different role in the other's. There are an endless conveyor belt of comic book movies at the moment, and it is only going to get worse, so we will reach the situation where almost every actor of note in the Hollywood circuit (except for Daniel Day Lewis) has appeared in one.
Secondly, Emily Blunt is one of the best actresses around at the moment. No disrespect to Mera or to Aquaman in general, but she should be given a meatier and more complex role. Any number of actresses could play Mera.