General:
* im- (implausible) - not plausible, unbelievable
* un- (unplausible) - same as implausible
* highly- (highly plausible) - very likely, believable
* more- (more plausible) - more likely to be true
* less- (less plausible) - less likely to be true
Specific:
* bio- (bioplausible) - plausible from a biological standpoint
* eco- (ecoplausible) - plausible from an ecological standpoint
* socio- (socioplausible) - plausible from a social standpoint
* techno- (technoplausible) - plausible from a technological standpoint
The specific prefix you choose will depend on the context and the specific nuance you want to convey.