lol my rules? I don't work for Innogames nor am I affiliated with them in any way. I'm a player.
But I can safely say this aspect of the game will not be changed any time soon.
Here are the reasons why:
1) This has been in the game for years.. There's never been any complaints about it nor has it diminished the gameplay.
2) If changed, it will unbalance the game. Imagine a city filled with hundreds of all kinds of mythical units... It would be impossible to penetrate. Hence - not gonna happen.
3) This aspect of the game is actually completely logical. Imagine, if you please, that we are in a world of Gods and war. Every God can command its own divine creatures. And this is where the problem arises - how is Zeus going to command Hades' creatures? Those are beings created for the purpose of serving their creator. Hence, any other God would not be able to maintain them.
4) Grepolis would lose a big part of its unique gameplay if this changes. It increases the strategic value when you need to think about what Gods to choose and what creatures to build in certain cities. If you can build all kinds of creatures in any city why would they belong to particular Gods at all? Just make the mythical creatures a separate unit type and that's it. Which would, like I said, take away from an important aspect of the game and make the gameplay more simplistic and boring.
I don't think I need to list any other reasons.
Thanks for reading and I hope you have luck in your future endeavors in Grepolis!
~Torgaddon