Personally, I love sequels to games - well, most of the time.
There is always a very high risk of ruining the 'game concept', or simply not spiritually following the concept enough.
An example here could be the Worms series.
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Worms 2 was a brilliant spiritual successor to the first Worms games - it had truly brilliant 2d graphics/animations, and I think we can all agree the cartoonish look is pulled off better in Worms 2 than most other games.
Worms: Armageddon didn't change a thing in the graphics - since they got that spot-on in Worms 2, and they recognized this, just like IV and your Darwinia/Multiwinia (I know effects etc. have had a major update, but the general visual setting is more or less exactly the same).
Instead, W:A introduced hordes of new weapons, as well as a very polished (at the time) online experience. It is a bit like 'Worms 2 Deluxe', but I believe they put enough new stuff in there to justify them calling it a sequel.
The first 'proper' sequel to Worms 2 was Worms 3D - complete overhaul of the visual style (moving from 2D to 3D, obviously), and - at least in my opinion - much deviation from the general concept of the game; shooting holes in the ground with grenades and bazookas. The third dimension did not do the worms series any good in my opinion, since the spirit of the game - as we knew it all the way back from the first Worms - had been tinkered with to such a degree that it was too different to be called the 'same concept'.
I'm sure the 3D Worms games are decent in and of themselves (I played a few of them with much joy) - but as a successor in the worms series, they just don't feel 'worms-y' enough. To me, at least.
Sequelising is risky.
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Now, a trend I see a lot in todays gaming industry is how everything is considered a
Sequel, and nothing an
Expansion Pack. A sequel that feels exactly the same (as in the worms games too, I suppose) can be quite upsetting - having to pay full price for two identical games with different content.
It usually goes something like "2 new levels! 5 new weapons! 8 new enemies! 1 new multiplayer mode!" - and they call that a sequel.
I see this especially much on the console market. It is probably very difficult to release expansion packs on consoles, but nevertheless, who wants to play, say, Gears of War when you can play Gears of War 2 - both are marginally different, gameplaywise, and you wouldn't feel like you wasted your money on GoW if you go out and get GoW 2 afterwards.
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The same goes - to some extent - for Darwinia.
I believe multiplayer was mentioned as a definite future patch possibility for Darwinia, yet here we are, at the verge of the release of 'Darwinia, the multiplayer aspect'. I believe Multiwinia would have worked brilliant as an expansion pack to Darwinia.
If nothing else, it could work like Warhammer 40,000: Dawn of War: Dark Crusade - it lets you play standalone with the two races introduced in Dark Crusade, and if you also have Warhammer 40,000: Dawn of War, it can be installed on top of that to allow choosing all races, as well as all the content/singleplayer stuff from the original - essentially merging the stand-alone expansion with the original game.
That way, I bet IV would also sell a few more Darwinia-games - to people curious about the singleplayer campaign imbedded in the same game.
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That being said, I don't really see Multiwinia as a sequel to Darwinia (unless it has a new/continued single player campaign), but rather the multiplayer experience expansion of Darwinia.
~ My two cents.