>>717252985
>80s or similar
>aliens are mysterious and we know jack shit about them
>the world musn't know, cover up everything
>start small
>base centric, not flying mobile command centre
>aliens are not dumb and trigger happy, they are calculating and they perform conspiracy theory level plots from the shadows so your agency has to uncover the plot as a main mechanic
>rival anti-alien agencies with a diplomacy system, ally them or become competitive, race to the saucer

That, or just give me what was the original inspiration for XCom, the UFO TV series but as a game.