I've been toying with this idea for some years now.
The party, preferably a group of space marine scouts or at least allied somewhere with a chapter, is sent to a remote planet on a scouting mission. Turns out there has been a war a few centuries back that left all the cities devastated, with a few chaos cults still in place, and maybe the odd survivor community. (Think Mad Max)
You could work out a few adventures around this, culmination should be that the cultists have decided to eradicate all non-believers and PC's and their allies among the natives are under heavy fire.
Somewhere along the road, PC's discover the remains of an ancient Space Marine Dreadnought. In itself a great relic, but under the circumstances, it might just save their lives and turn the tide.
Fun part now would to see if the space Marine inside is still functional or does the metal housing need a new spirit? Maybe it's tainted by chaos. Maybe the taint will only slowly manifest upon return to the chapter,...