No movie like Trek is going to sell without a plotline, you can't have a proper plotline in Trek if there's no bad guy.
What options is there if there's not going to be a bad guy?
What action could there be without a bad guy?
A movie's antagonist doesn't
necessarily need to be a specific person with a specific, morally questionable motivation.
An antagonist can be drawn as a more abstract threat.
Still maintaining tension, but not being a "bad guy" exactly.
A villain could be a specific person (Khan, Admiral Marcus, Nero, Shinzon, Ru'Afo, the Borg Queen, etc) who is attributed as being morally questionable, against the purity and 'goodness' of the protagonist characters.
A villain could be, though, an abstract concept as well. And it often was, in TOS the television series and occasionally in the TOS movies as well.
THE MOTION PICTURE has a villain (V'Ger) whose motivations are not particularly evil, but who is still a threat to the Federation despite not being a 'bad guy' per se. THE VOYAGE HOME, likewise, has got a probe that threatens Earth, despite not in itself being a specific person, although it has got a specific motivation (find the whales).
THE DOOMSDAY MACHINE, one of TOS's most lauded television instalments, features a large entity of sorts whose motivation is to be self-sustaining, to gain 'nourishment' to stay 'alive', which it just happens to do by destroying planets and starships. The fact that the titular doomsday machine isn't a recognisably humanoid 'villain' (heck, the fact that it arguably isn't even *sentient*, as such!) doesn't lessen it's impact as a villainous force that the Enterprise needs to Stop At All Costs.
When people call on the next movie to feature something other than another Khan/Nero/Shinzon/Ru'Afo/etc style "bad guy who is after REVENGE", what they're really asking the producers is to be more creative, to use Star Trek's format in a way that it has been in the past, to portray a wider galaxy than just out own little corner of it. Give us something new, something different. Strange new worlds, new life, new civilizations. New motivations and new threats.
Believe it or not, it
can be done.
Or they could just take the easy way out, and have the Enterprise fighting off a fleet of warships commanded by some moustache twirling villain out for revenge. Yawn.
