Okay this is sort of a mess because Im referring to four different worlds here .... an early childhood writing which combines science fiction and magic, a teenage writing which was much the same but had teens instead of young kids as the superheros .... a later teenage writing where i STILL had magic and sci-fi elements mixed but mostly wrote about large numbers of people .... and finally my current world which takes place in a low fantasy world with absolutely no magic at all.
Im just writing this up because I felt like it .... showing that some of the ideas can in fact be justified .... without saying that any of them are specifically good or bad plot ideas.
A sexually selective plague kills off or sterilizes almost all of the men, or almost all of the women.
I basically did this in my teenage writing but it was a minor plot element since it happened in a place out of reach of the superheros and they were neither helped nor harmed by that distant war.
Creatures from our mythology (e.g., centaurs, dragons) occur among the wildlife native to an alien planet.
The definition of "dragon" is pretty vague, so I think this is viable, but I suppose it would be odd to have a planet in which there is only one dragonlike species and nothing else like that.
The evil duplicate of the hero, sidekick, universe, etc.
I got the idea for this from Zelda II .... but later I separated the character from his duplicate and only the name remained.
Societies wherein gender roles and attitudes are completely reversed.
Women are taller than men on much of planet Teppala, so this idea is one that I'm still clinging to, and will never give up, although I'd say that the balance of power is reversed, not the gender roles .... women are still doing most of the childcare, for example, and men, despite being physically weaker, are doing most of the fighting in war. Thus this is the only idea on the entire list that still applies to my current writing.
Men and women live in separate societies (and I'm not talking Mars and Venus, either).
This idea is another one I still cling to, .... although i came up with the idea when i was still writing science fiction, i've still maintained it even in my low fantasy world .... for the Crystals, and a few others, I want to have literal male and female nations, although nothing in the plot depends on this, so if I concede that it's not viable I can reduce it to a legal formality and say that men and women live together but obey different laws because, formally speaking, they have citizenship in male or female nations respectively, even if the nations' territories overlap. (And men vote for men, women vote for women, etc.)
Children with access to the highest levels of military planning, scientific research, and governmental decision-making.
The gang of cute and/or misfit kids rescue the universe, where a large group of competent, organized and well-armed adults failed.
The aliens' plan to exterminate the human race is stopped at the last moment when they notice a human exhibiting some virtue, such as love, humor, etc.
All four of these are okay if the children have magic powers, particularly mental powers, as the child superheros of my early writing did. It didnt even occur to me that it would seem odd to have children running the country and international (or even interplanetary) political events .... given that, with their mental powers, they are smarter than most adults, why wouldnt they be in charge? That said, much of what appeals to me about my early writing is that I made it up as I went along, and the superheros were prone to impulsive decisions, such as in one case singlehandedly canceling a major war because they befriended some young kids on the opposite side.
Although humans still have multiple languages, each alien race has only one langauge.
The entire population of the planet lives in one city.
I think these are believable .... I did this on purpose in my early writing for the spiders of planet Theta, and said that they were by nature more centralized than humans are, and therefore had only one language and one large city (though the rest of the planet still did have some settlements). In fact it was a plot element .... the superheros knew that they could easily find their way around the unfamiliar planet because there was only one city and no reason for them to look elsewhere for their treasure
Inherited supernatural power (telepathy, lycanthropy, etc.) becomes pronounced at the onset of puberty.
I was working with this idea for a while but essentially abandoned that whole storyline. One corollary though was that children were also immune to magic, which allowed me to somewhat keep alive the universe of my childhood writing.
Humans leave for the stars, forget all about Earth, and rediscover it later.
I never decided on an explanation for why humans were living on so many planets in my writing, but one idea that I'd had (though it makes no sense, I know) is that humans originated on some other planet, and migrated to many planets across the universe, of which Earth was just one. Or that humans did originate on Earth, some subset of humanity developed space travel, and lost contact with the rest.