Are there any plans to make a sixth installment in the Heroes series?

The maps are very boring and monotonous.

Download maps made by mapmakers, there are many of them;)

There are few monsters.

How many more should there be? :smile07:

There are few spells.

Not that few, and almost none of them are repeated.

Large maps with dungeons are unplayable because the "computer" starts lagging after a couple of months.

Did you mean very large? I only experienced lag on those with dungeons after about 3 months, not sooner.
So, a lot of this is exaggerated. And the graphics aren't anything special. It's decent, but nothing more.