Reading up on a world with a SO2 sea and by extention a SO2 atmosphere. There would lots of extra sulfer as a sink for oxygen with some of the shores being "beaches" of sulfer.

A possible life cycle could be plants creating SO3 (thru photosynthysis) and animals that convert SO3 back to SO2.

Most likely no higher animals. No free oxygen would mean no fire. A would be a civilization would be trapped in the stone age even if there were sapients.

As a double whammy there would be no metals. copper would probably be abundant but being mixed with sulfided there would be no way to separate it without fire. Metalurgy would be DOA on this planet.

How to incorporate a sulfer planet into the RPG setting?

One idea might be that explorers put down to investigate signs of life and since their lander is a chem engine powered craft can it then take off if its engines rely on combustion? (I dont believe we've ever specified what exactly is the chem fuel used in the setting) but many (most?) rocket fuels do include oxygen components so perhaps this isn't a problem. However, sulf compounds can be corrosive and suppose the first scout team dallied too long and certain metals of their craft were eaten by the environmental sulfides resulting in exposure of fuel. oxygen reacting with sulfer and so on as well as loss of craft integrity requires a rescue mission.

Naturally you'd want to send the rescue party in a better craft but even then the clock might be ticking on the corrosiveness of the environment putting the PCs under pressure to accomplish all of their mission goals. Thus you'd need to specify how long a craft would maintain integrity.

Goals:
1. locate and rescue first party- complicated by the fact that one member of the party has wandered off and must be located.

2. Leader of scout party insists that samples and data must be secured as well. Is it dangerous to enter the first party's craft? why? loss of structural integrity, sulimal lifeform has entered and doesn't know it cant eat the PCs?

3. in searching for the lost scientist the team locates a derilict craft (to explore it or not to explore it?- mini dungeon crawl and just what is the purpose of that and what does this side trek ultimately lead to?)

4. earth quake damages rescue craft? requires some repairs and the clock is still ticking on the corrosion problem.

Possible classifications of animals on a sulfer planet:
aquatic forms are classified as "sulfish" instead of fish (sulfer + fish)
sulfants (plants)
sulfimals

sulfimals might not realize that they cant digest the visitors but not for lack of trying.