One way you can do a "remake" that is still a sequel is to just have parallel universes the way Doom (2016) did it. No reason you couldn't have a remake that reuses a lot of locations in modern graphics and has a lot of the original gameplay elements with differences justified in "this isn't the same timeline as the original game!".
I always liked the little rapid piano effect in the Guardian Of Ice music, like chattering teeth. It works great for an ice-themed map. There certainly are modern ports for playing on modern systems, like GZDoom. And the game lives on in the community too, like at Doomworld; even now I'm trying my hand at making some Hexen maps. Doom gets most of the attention but Heretic/Hexen can hold their own! 🙋🏻♀️
The best way to do a follow up to Heretic and Hexen these days would not be to do a follow up to Heretic and Hexen, it would be better to go straight to the themes of the era that inspired the game in the first place; The Dungeons and Dragons substrate, the artwork of Gerald Brom, Boris Vallejo and Frank Frazetta, the dark fantasy movies of the 80s and 90s like Conan, Hellraiser, Labyrinth, Dark Crystal, Army of Darkness, with their stop-motion horrors and surreal, forbidding landscapes, none of which was even remotely possible to emulate on the computer tech at the time, but left a massive imprint on these games nonetheless. No one seems to do that kind of thing anymore, so even though the technology has easily surpassed the level it would need to be, that style just isn't 'in' I guess.
