forum sci-fi masters, please help with alien motivation for coming to earth

Started September 13, 2018 by @mckapo

tune

Here is what I have so far, plot wise:
Aliens, on an intergalactic mission to find worlds able to sustain their cursed biology (backstory for this), find Earth [think 5000b.c.] and become friendly with the native humans, who worship them in return for the technological advancements that help create numerous religious works. During this time, the aliens try to change their dna in an attempt to breed with humans, hoping that they can have offspring in case they are not able to find a planet for their species to survive on [they have masks over their mouths, as they emit poisonous fumes that are toxic to humans and the planet]. They are successful in their attempt at interbreeding, and eventually move on.

In 2105, the Vamette Flower lands on Earth, breaking through it’s asteroid encasement and taking root immediately, growing to a substantial size in just a few short years. As the alien artifact is studied by scientists and the like, the human population goes into a panic.

In 2113, the Vamette Flower blooms and it’s pollen is spread throughout the entire world. This affects all descendents of the alien/human hybrids that were created thousands of years ago, the Flower activating the alien bloodline and turning the humans into Toxins (what the hybrids were always meant to be), which restarts the mass panic that went on 8 years earlier with the arrival of the Flower.

In 2163 (start of story), the Earth has dissolved into mass war and hatred, the Toxins waging against the humans, as for the past 40 years, Toxins have been treated as less than shit, branded, marked, stowed away, and they have had enough of it. Technology has stagnated, and fearing for their lives, humans have released radiation weapons that plague humanity.

It is at this time that the aliens have decided to come back and see if Earth is a suitable replacement for them. This ignites a rather delicate treaty between Toxins and humans as they fight against the aliens for their home.

motivation for alien race good enough?
does this sound even remotely interesting? or boring same-old same-old?
if so, any ideas on how to do something similar to this, or even completely disregard all of this and create something else

Back when this was a baby idea, I had/have two MC's that are Toxins and a human, that were all government workers, and this probably would have taken place a few years after the Vamette Flower bloomed, with the government-vs-rebels kind of plot.

But, I wanted to go BIG, so i'm thinking…more moooore sci-fi, let's bring in the aliens, and all that stuff.
but now i need a semi-decent plot

My question is, why did they leave in the first place? If their goal was to "find worlds able to sustain their cursed biology", and then they successfully breeded with humans, why would they leave? Or at least, why would they all leave? And for so long (side question: how long is the lifespan of these aliens?). I think, logically, they would try and colonize the planet after learning it was viable. Or maybe take some humans with them when they left?

And how is their biology cursed? Are they just toxic creatures or does this 'curse' eventually lead to the extinction of their species? Because I could see that as a real good reason to travel the galaxy, bangin every alien you find to try and make a baby.

Also, if you wanted to go crazy big with the aliens, they likely tried to breed with other alien races, so maybe they come to earth with other hybrids? Or other hybrids join with the humans and Toxins to fight this alien race?