Making it a stealth based game would be key
When it first starts up, it takes the unknown, survival horror type atmosphere. You've got no weapons, no idea what just happened, and now you're trying to remain incognito to a far more superior being than you
As it progresses, you meet up with a few other stragglers, survivors, who know a little more than you. Eventually, as you encounter enough people, you start to essentially build a small resistance group. It then comes down to finding weapons
So? It goes from a survival horror, to a survival horror treasure hunt, and once within the safety confines of say, a military base or bunker, the plot develops as you notice that the abandoned bunker housed knowledge on the enemy
Yourself, and the few other people you encountered, then devise a plan to ambush the enemy that you've been keeping away from all this time using this new found knowledge.
Note: If you're unsuccessful from hiding up until this point, you don't actually die, or fail. You end up being herded just like everyone else to a certain location, thus bringing you to a different style of play, a more "The Great Escape" sort of idea. This opens up different story branches as you'll never encounter, or encounter again, the very people you would have had you stayed hidden.
So if you and your group manage to take out the tripod in a successful ambush (you'll have to infer from the information gathered at the bunker on how to do it properly), it becomes something where you end up trying to figure out where these things are taking the people, and why. However, at this point in time, the game will have to introduce new enemies, foot soldiers perhaps. You're still outgunned, but can put up a decent fight if need be. This is where stealth becomes a priority, and engagement to a minimal.