The weapons are visually and mechanically different from Doom's, as are the enemies.
The weapons are very similar to Doom, they are not exactly the same, but e.g. your staff in Heretic is the fist in Doom, the wand is the pistol, the red glove (or whatever it is) is the shotgun, the electric gauntlets is the chainsaw, etc.
That's not really the case IMO*, but again, I don't mind games that lift mechanics from other games, and such a thing is obviously inevitable in this medium. It's not the mechanics or gameplay that concern me (and those are obviously different given the new combat system they're trying), what confuses me is why this new project is modelling itself stylistically deliberately on Fallout to the point where - by the devs' intent - you could look at a screenshot of it without context and actually think that it literally is a new Fallout game.
I wouldn't have a problem if they did something actually analagous to what Heretic is to Doom - essentially, making a new original game on the Fallout engine that used Fallout's own mechanics as a base but was a totally different game content-wise with a new setting, visual style, interface, etc. In fact, I'd be thrilled and hopeful that it heralded a wave of "boomer" cRPGs.
Well, that is where "inspired from" comes from and people do like playing and making that sort of stuff - games like Alien Breed on the Amiga was basically Gauntlet with an Aliens skin and, talking about Aliens, fangames for existing franchises existed for decades considering that one of the first mods for Doom was Aliens.
If they were making something intended as an actual Fallout fangame (in the vein of Resurrection or Nevada), I wouldn't be complaining. Just like I don't complain when Half-Life mods are set in Black Mesa, but I
would be baffled if New Blood announced a new game called "Half-Life Inspired Project" about a man named Gary Freeman who uses a trademark red and silver crowbar to fend off deadly headlobsters while escaping the crumbling Black Mountain Research Facility.
So i don't see how it is hard to accept that some developers may want to make games similar to stuff they already like, this is something that many developers always did for a very very long time now - hell, even since the beginning of computer games, people were making games very similar to existing games all the time (as an example see the original Adventure game and all of its spinoffs and imitations).
Right, but there's inspiration and then there's whatever this is. Hedon is inspired by Hexen, but doesn't ape it. Age of Decadence is inspired by Fallout, but doesn't ape it. Fallout itself is inspired by Wasteland, but doesn't ape it. You could easily - and people have - make a "Fallout clone" that plays near-identically to Fallout without directly imitating all Fallout's aesthetic elements and visual design and interface (down to the exact same font, which I still can't get over). It could even be retrofuturistic and post-apocalyptic and obviously Fallout-inspired if that's what they want to make - Wasteland 2 and 3 (for all their problems) managed to do exactly that without ever coming across as a bad copy of Fallout. The whole point of this game's style is to nudge you in the ribs and say "remember Fallout? remember Fallout?". To which the answer is obviously "yes", but I don't want to be reminded of Fallout, I want them to show me something entirely new.
*putting this down here because it's tangential to the main conversation, but I always feel like Heretic's reputation as a straight-up Doom reskin is unfair - it's true that the gauntlets are a direct reskin of the chainsaw and the wand is very much the pistol, but the rest of the weapons don't
really track onto Doom's, especially if you consider the Tome of Power alt fire modes (ie the Phoenix Staff turning from a rocket launcher into a flamethrower, and the shitty mace thing sending out giant bouncing spiked cannonballs).