HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Maybe it was suggested, but I didn't see it. Could you add crypts to Inferno and Citadel? Everyone else has them, and they wouldn't hurt here.

This would already be a change to the core gameplay, and it's unlikely that the gaming community would agree to it. Barbarians don't build crypts – they either throw the corpses into the water or burn them because it's impossible to dig in the rocks. That's why skeletons lie around there, uncollected (this is your substitute for crypts – balance). Well, accordingly, you can't build crypts on lava either – it's lava, after all. On Inferno land, if it's generated correctly, there are more free piles of gold than on other types of soil – that's also balance.
The only thing that wouldn't hurt Inferno is to fill in the native soil underground on its own territory, because there's no bonus there, and more often it's an anti-bonus in the form of blocks and labyrinths.