What is your opinion about Heroes IV?

That's precisely the problem. After the unsuccessful release (I can't bring myself to call it a failure) of the fourth installment, the entire series revolves around "how can we add something new to the framework of the third game?" In the fifth game, this still made sense and looked decent, but 10 years have passed, and nothing has changed.

Personally, I would have ended the entire series with the fifth installment. Instead of making a sixth, seventh, etc. (maybe they'll come up with more?), they could create high-quality expansions and collections of campaigns/scenarios for the fifth game. And, by the way, people are still actively creating custom maps for the games from the third to the fifth, which is a great tribute to the mapmakers.

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Moreover, things are going backward when the series director announces Heroes 7 as "a collection of the best songs of an ancient, well-known rock band." The modders will clearly do a better job, for free, and give it away to the people.

There's another nuance regarding the seventh game, and it's quite important. It's about the system requirements for computers. Not everyone has hardware with such parameters, and buying a new computer for each new installment sounds a bit absurd.