I figured I might as well bang out a little summary of the different flavors of videogame "sequels" that exist. It should help us a lot. Note it will be stream-of-consciousness type stuff, so it won't be particularly well-written. But it will be a good start, and I'll anyway eventually turn it into a proper article.
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1. Not sequels
These are sequels in name only. I.e. they are completely different games from the original, which, however, for purely marketing reasons have been slapped with the name of a popular game. They might be dressed up with some elements of the original game, but these are mostly of a secondary or merely aesthetic nature. Games like these do not get a free pass on account of their "predecessors": they will be considered for inclusion purely on their own merits, and, if included, will take up a brand-new slot.
Examples: Metroid Prime, Neo Contra, Command & Conquer: Renegade, World of Warcraft, Halo Wars, that Game Cube Wars game by some British dudes, Baldur's Gate: Dark Alliance, pretty much every 2D action game that was translated to 3 dimensions and every 3D game translated to 2, etc. etc.
2. New engine
These deserve the name "sequel" more than any others. They have brand-new engines, and as a result usually play and feel quite differently from the original. So not only are their scenarios different, but the mechanics as well, to a greater or lesser extent, depending on the game. These should be considered on an individual basis. If they play very differently from the original then they may deserve an entirely new slot by themselves; if they do not play very differently (because, for example, the engine was used mostly for visual upgrades), then they should be considered for inclusion
alongside the original -- i.e. occupying the same slot.
Examples: Half-Life 2, Deus Ex 2, The Super Shinobi, Contra Spirits, WarCraft II, WarCraft III, StarCraft 2, Baldur's Gate 2, Bare Knuckle II, Bare Knuckle III, The House of the Dead 4, Time Crisis 2, 3, 4, Project Gotham Racing 2, 3, etc.
3. New scenario
These usually (but not always) retain the same engine. Apart from that, they are either exactly the same as the original, rules-wise, or introduce only slight alterations. The main reason for their existence is to give the player a new set of stages to trundle through. If the original game was great, they too will be great by definition, except perhaps if the rule changes really suck or if the new stages really suck. These games, if they are included, will ALWAYS be included
alongside the original game -- i.e. they do not deserve a new slot. They are basically expansion packs for the player who has finished the original game and simply wants more. If he doesn't want more, fine. That only means he doesn't want more -- it doesn't mean the "sequel" sucks (because it's not really quite a sequel).
Examples: The Super Shinobi II, Gears of War II, Shin Contra, Metal Slug 2, 3, 4, 5, 6, all Wars games and all Fire Emblem games, Icewind Dale II, Golden Axe II, Sonic 2, 3, all the GTA III games, etc.
4. Minor rule revisions
These come in different varieties. Your lowest kind are basically patches. Every patch modifies at least one rule, so it results, basically, in a "sequel". The thing with patches is that they are not supposed to ADD new stuff -- just to fix existing problems -- but there are plenty of examples of patches which added significant new features -- for better or worse. These are free, which is why no one regards them as sequels, even though some of them have more of a right to that label than sequels you have to pay money for.
Then you have stuff like Virtua Fighter 5 Ver. A, B, C, D, etc., which are a step up from patches because, though they may fix bugs too, their stated objective is not to fix bugs but to ADD new stuff. But again there are no black and white distinctions: there are patches that add more stuff than new versions, and new versions which fix more bugs than patches. The other thing about these new versions is that they are also usually free (though perhaps not for the operators).
Next step up is upgrades of the kind you usually see in competitive multiplayer games such as fighters, for example. The super turbo kind of deal. All of these, together with patched versions, and updated versions, etc. etc. are automatically included alongside the original game, as long as they don't fuck up the game too badly (which they usually don't).
5. New skin
This is a weird category. Off the top of my head I can only remember one such case:
Uo Poko and
Mushihime Tama. (Note: I know the site layout in those links is screwed right now -- I am working on fixing it).
The thing here is that these two games ARE the same game, the second being a sequel to the first in all but name (and of course theme, aesthetics, etc.) They play so similarly that if I decided to include the first, and if I deemed that the small changes introduced by the second were not detrimental in any way (which they aren't), then I'd be obliged to also include the second one --
alongside the first.
(Note that this is an example -- these two games are not really good enough to be included.)
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So yeah, I think that is all. Have I missed anything? I think all videogame "sequels" (and even games which are not officially called sequels but in fact are) can be more or less neatly fitted into one of the above categories. This would then make it easier to determine whether a "sequel" deserves a new slot or not.
All the above are just basically general guidelines. At the end of the day you have to consider each game on an individual basis. What you need to decide is whether the experience of playing the sequel is
different enough so as to deserve to take up a brand-new slot. Most simply don't.*
Meanwhile, you have the marketing people doing their best to confuse you with all sorts of unnatural naming conventions and dirty tricks.
*and that is in no way a bad thing