A branched campaign would simply shorten the story line as there are only a limited amount of maps a developer can make.
I like the try-again idea, it’s the singleplayer way and needs to be there if you don’t want to make a distinction between both, you’ll have to go with the try-again, without it it would give a very lame singleplayer experience.
It also really makes winning the map matter more. If you don’t want to spent your day playing the same level over and over again you’ll really give more then when the maps would change anyway.
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