Ahem, no. Not the case there. The artwork under OpenTTD fails under fair reimplementation for cohesiveness with the current extensions and modules. Ditto with FreeDoom with Doom: is not inspired but art-compatible so your Strain, Requiem, Back to Saturn and so on PWADs run the same without texture or styling clashes.

Artistically speaking FreeDoom it's closer to Half Life and the like than Doom but here's the catch: playing Strain for instance won't look like a mess, but different, a bit like a demaked Half Life (or a game from its era with the Unreal engine) but not a copy.