Enlarging Deutschland leads toward Scharnhorst

Could the Deutschland class be lengthened and broadened into a roughly 25,000-ton design?

Yes, but that development path effectively produces the Scharnhorst concept by extending the hull, adding a third turret, and changing the machinery.

Lengthening and broadening Deutschland would create room for another main turret and a more capable machinery arrangement. Those changes do more than improve the original armored ship; they move the design into a different displacement and combat category. The historical Scharnhorst class is the practical result of following that path. Incremental requirements can turn an enlarged cruiser concept into a small fast capital ship.