"Heroes 5". But are they really "Heroes"?

One very large add-on is not the solution. If you proceed in this manner, it could lead to even more confusion. However, it is quite possible to release a single campaign that serves as a prequel. It should detail the origins of Sandro and explain why Crag Hack frightens people so much, and cover many other similar aspects. Alternatively, as a last resort, you could remove all such references from the fifth installment to avoid confusing the audience.