We can create a d20 superpowers but we shouldn't adapt the superheroes, not without a previous nerfing. My idea is to allow a Superman defeating a dragon or a giant but losing the combat is also possible.Do you remember DC vs Mortal Kombat? If you played Superman you could lose the fight. Winning a combat without effort is too boring.
I imagine the setting like a century after of surviving a "War of the World", a failed invasion by the tripods, and the technologic advanced was stopped, and some economic crash after a couple of epidemics. The classis D&D spellcasters are almost forgotten and replaced by "magictek", too expensive for the mass production but very useful for the right place and time (for example to create machines what produce XXI century materials). One of the main antagonists is the deep state, a parallel secret agency, a hidden lobby, ruled by no-humanoids...The lord feys are trying a "teological revoluntion" against the pantheons, the gods who defeated primal titans for the ancient titanomanchy. (this is now our real world but a fictional setting). And then a planar gate is opened, or discovered, to another pos-apocalypse world, more advanced, but also damaged for the last war by the MDWs. Later new planar gates are opened, sometimes the first encounter is hostile, but other times both are peaciful and diplomatic (for example trading salt and exotic species).
There are bulletproof suits, by skin of near exctintion species, and its hunt is illegal. Most of criminals would rather no-lethal weapons because in this world murder victims can come from the death to take reveance. Even the firearms in the battlefield are almost a religious taboo and created more to hurt than to kill (sometimes war gods punished the firearms in the battlefield sending petitioner warrior souls from the Walhalla with bulletproof traits).
