Be massive avp fan

>be massive avp fan
>want to put xenomorphs, predators and assorted Halo and Mass Effect ayes in a campaign as a potential player races

How could I pull off a system like this without it being a steaming pile of shite?

>xenomorphs as a player race

Absolutely not. The rest is fine if your players are willing to be a little fan fictiony and you make the appropriate technological changes.

>How could I pull off a system like this without it being a steaming pile of shite?

By doing a good job :)

Look, you either have to find a plausible narrative which supports combining the elements you want to use, so that their combination doesn't throw people off. Or you have to make the setting so over the top and fun that people ignore the way it's elements don;t really fit together simply because it's entertaining.

Basically you have to write it up really well, or rely entirely on Rule of Cool.

they already covered this in the comics.
Synths designed to look like aliens.

This. Xenomorphs are not player characters, they're "rocks fall everyone dies" in sci-fi settings.

Might as well throw up some ideas for this campaign

Of course it's set in on a galaxy wide scale, involving different locales on different planets. However, the alignment system is on a totally different vain. Lawful to chaotic is still there, but instead of good and evil, you align to one of four factions, Alliance, Covenant, Elder and Independent:
>Alliance are the core group, with mostly ME races with a sprinkle of humans. These are the main dudes you'd go for if you want a "good guy" character
>Covenant are a religious military group who uphold a strict code of hierarchy and honour. While most are human haters it wouldn't be uncommon for them to work with smaller groups (ie the players)
>Elders are a predator led network who seek power through planet conquering, and is composed of most of the galaxy's "asshole groups" seeking to overthrow the two core groups
>Independent consist of races that don't conform to these three (see Quarians or Xenomorphs), or individual rogues that do. This would be the group that the players likely would start with if they want to be on each other's good (or potentially bad) side

Other than those big changes it's most likely a d20 system with custom races and classes to suit the setting, and a story to boot

>How can fan-fic not be a steaming pile of shite?
It can't.

You can pick one franchise and theoretically turn it into a decent table-top. Because then it's a translation into a new medium, instead of fan-fic. Writing your own setting that combines franchises? Well, it can either be a fighting video game, a racing video game, or fucking garbage.

Pretty easy. Every time a xeno takes melee damage, roll to see if the opponent gets damaged too.

Whatever man

A lot of the races could be put into 3 basic categories; Social, Warrior and Intellectual.

>Social races use charm and people skills to make a name for themselves and get into the good books of others, or help escape sticky situations. Most social races end up being the core leaders of their groups. Most would use their wit, charm, political intrigue and sometimes pure shit talking to climb the social ladder. Expect Charisma/Wisdom bonuses

>Warrior races are, as the title suggests, species who's core values are forged in combat, and whether is based around honour, pride or pure blood fuelled instinct depends on the species. They make the muscle and military might of most groups. Expect Strength/Constitution bonuses

>Intellectual races are the backbone of most groups' technological prowess. Their advancements in travel, medicine and weaponry means they are normally the ones providing the tools for other races to advance under their influence. Some races mix well in their factions, while independent species stick to their own groups and advance on their own. Expect Intelligence/Dexterity(AC) bonuses

Of course there are mix and matches with other species, and some don't fall into these categories, but it provides a base line for how the character might be seen in certain situations, and how they might handle them