Just catching up as I missed several days

31
- This game launched a major franchise, but is not currently available to buy.
- Whilst many of the later games in the franchise licensed songs to use, this game used songs written and recorded by members of the development team.
- Was successful enough to receive an expansion and a sequel, but it’s once the series made the move to 3D that this franchise really exploded in popularity.

30
- Generally considered to be more difficult and unforgiving than earlier entries in the series. This is especially true of the console version, due to the dark visuals and the save system used.
- You’ll always start and finish the game in the same locations, but the order in which you tackle the middle sections is up to you.

29
- This ‘trophy’ of sorts you get by defeating a powerful enemy was once required to progress through the game, but now it’s just a means to make some money.

28
- We’ve been in these places before, but the perspective is now a little different.

27
- Whilst conservation of ammo was a feature of the first game in the series, in this sequel you’ll have a supply that is pretty much limitless.

Current list - https://gamrconnect.vgchartz.com/thread/247804/the-official-thread-the-14th-annual-greatest-games-event-post-only-once/4/#3

#31 - Grand Theft Auto? Was surprised to see it listed on Steam but unavailable for purchase.

#30 - Tomb Raider III maybe? It was brutally difficult without a guide back in the day - I never did manage to get very far in that one. I definitely intend to fix that when the remaster package comes out next year.

#28 - Resident Evil 3?

#27 - Resident Evil 2? Got too hung up on the sequel part, assuming a direct sequel. The limitless ammo sounds more like Resident Evil 4 though.

23 is Super Mario World?

21 is No One Lives Forever I think?

25: Soul Calibur

31 - correct. Not sure why they don't have the first two GTA games for sale. I'm sure they'd sell a fair amount on name power alone.

30 - correct. I first played TRIII on the PS1, and yeah, it was pretty brutal. Having replayed the PC version on Steam, it's quite a bit easier due to the save anywhere system.

28 - incorrect, but not far off

27 - incorrect, and whilst not as close as your guess for 28, you're still in the right sort of ball park.

21 is absolutely right!

23 is almost right, but I made a hint that could apply to a bunch of games in the series, so that was mean. It's not World, but...

28 - This one is probably Resident Evil 2 then?

So 23 could be.... Super Mario Bros 3 maybe? The Koopa Kids were in that, but were they named at that point?

#24: Super Smash Bros. for Wii U (Bayonetta being the culprit here, I'm guessing Steve is the overpowered one in Ultimate)

Yes!

But they were named already back then, somewhere, like in the manual or the guide or something, weren't they? Now I'm not sure... :-|

You've taken the lead in guesses with these two in any case.

Alright then, let's continue. First hints for the next batch;

#25: The original game in this series was named after the evil weapon, but this well-received sequel, and the series thereafter, was named after the good one. Guessed by Darashiva - SoulCalibur
#24: This game got two expansion packs, but the second one in reality doesn't actually have anything to do with the main theme and historical era of the main game.
#23: Features bosses that are named after musicians. Guessed by drbunnig - Super Mario Bros. 3
#22: In this game's predecessor, this unimportant side-character comments about how a certain item may make him "look a little tougher" when you give it to him. Who knew though that in this game, he'd become a lot tougher when he dons a similar, but much more powerful item.
#21: At first the designers of this game had a male protagonist in mind. But after they noticed it got a little too much like a certain famous spy-movie franchise they changed the protagonist to female. Guessed by drbunnig - The Operative: No One Lives Forever