Changes

So you have decided to use aliens in your game. You can finally play out battles in space and everything, but what if your campaign is pre-existing and you cannot start out with the knowledge of aliens in the first place? You may decide that you wish to start introducing the aliens subtly, perhaps beginning with an abduction, or strange sightings. The PCs could discover ancient ruins which ends up projecting a beam into space and weeks later, discover it has called something down from the sky. This brings of the questions of how they will interact: will they be benign or vicious? Why have they come? Do they bring their entire force of a small scout ship? More importantly consider the posibility possibility of your PCs getting a hand on alien technology. You may wish to handwave it, saying it cannot be used because it is too advanced, and thus also provides a fine way to pose a powerful challenge against equipped opponents, yet not ruin the expected wealth by level by allowing them to loot the item-heavy bodies of their enemies. Alternatively you may want to allow them to use alien items, all the way up to starships. If such happens, your campaign setting may expand to other worlds, providing an even larger experience.

Politics, action, war, even ship-sailing themes can be explored using alien technology, as the PCs deserpately desesperately attempt to figure out how to fix their ship as hostile beings chase them down, or their diplomacy attempts end up causing a much larger problem for their home planet. They may reach another land filled with alien gods, or end up hurling through time, only to arrive back home in the far future. The choice is up to you!