Resources - Changing the hiring cost of creatures.

In my opinion, it would be better to increase the demand for Mercury, Sulfur, Crystals, and Gems by increasing the cost of construction and significantly increasing or making it so that mines produce resources not every day, but once a month!

Once a month? ...that would be hardcore. It might be interesting to play like that sometimes, but only for variety! :) Such an innovation certainly won't please everyone :o, and besides, the game might become too long, or players will complete it without having time to build top-tier structures. In this mod, problems might only arise with upgrading creatures or the inability to buy out all the elite units before the end.

Quite a few mods or mod ideas come from people who prefer to make games longer.

In general, if you capture all the necessary mines in time, the game won't become too long. Besides, markets still exist; sometimes it wouldn't hurt to buy the missing resource.

Creatures traditionally require gold, and construction requires resources + gold, and why erase the difference between these two aspects of the economy?

I don't want to argue, but you know that this isn't entirely true. What about level 7 troops? Why do they also require resources? Dragons aren't buildings. Why should they necessarily be different from troops of a slightly lower level? If you disagree with the HoMM developers in this case, that's another matter :)

And to prevent the game from becoming too long, it might be worth reducing the amount of resources required to build creature dwellings along with these new features.

Perhaps. But this needs to be tested, because it's not entirely clear how much the cost of building dwellings should be reduced, and of course, there will be many who disagree ;)