If GOG ever gets Heretic II, will users who already purchased the Heretic + Hexen Pack also get Heretic II added to their libraries?

It's difficult to say, that's depends on the publisher. Hope one day that game arrive here.

I would personally say definitely no, and at most anybody could say is "nobody can possibly know".

It may get added to the pack, but not retroactively. At least I do not think it would.

Unlikely, but you can keep fingers crossed. Heretic II was published by Activision, whereas Heretic and Hexen were GT Interactive/ID Software and the main reason you've not got Heretic II here is the same reason that Wolfenstein (2009) isn't here - a rights quagmire.

It's an interesting game though - although not helped commercially by coming out around the same time as Half-Life.

Heretic and Heretic II were handled by different publishers so not likely unless Bethesda/Id Software have acquired the rights to Heretic II from Activision.

Highly unlikely.

Highly unlikely. If anything, the current bundle would get pulled and a newer, more expensive one which also inluded Heretic II, would take its place

I never played it, but it does seem odd that they completely changed genre.

Owners of that bundle could possibly get a discount but it wouldn't matter to me if they didn't. I'd buy it either way.

Certainly not. And If GOG ever gets Hexen II: Portal of Praevus expansion - current owners most probably will not get it either, but a 15% discount might happen.

Bethesda (Zenimax, now subsidiary of Microsoft) and Activision (subsidiary of Activision Blizzard) are different entities, and I don't recall Activision being too generous before, look at their prices and remember when was the last time they offered good discounts.