I had posted in two other threads my thoughts about DLC's coming up. Most of them involved atmospheric landings of one kind of planet or another (gas giants, icy, lava worlds, high pressure worlds, water worlds) and possibly one DLC involving legs. My guess was based on FD being able to create content for each type of world, then put it up for sale as a DLC.
Not everyone wants legs. Fine, Don't buy that DLC. Not interested in icy worlds but can't wait to dive into a gas giant looking for space whales? Fine - get THAT DLC instead.
I'm guessing they'll be released one at a time with the last being Earth-like worlds which will obviously have to be populated with huge amounts of things that might want to eat you (or crawl up your nose and eat your brains).
Legs (if you want that DLC) could be incorporated into any of these DLC's (well, except gas giants unless we're talking about an EVA jet-pack suit). New SRV's could be created to do creature hunting or setting up mining facilities on the ground.
Free updates could include things like newer life forms here and there on various planets, as well as improvements in black holes, etc. I'd love to see Earth-like worlds with civilizations on them (not landable) that we might make first contact with. How they react to us in the future depends on how diplomatic we are in our first contact.
BTW, for landable Earth-likes with advanced civilizations on them, I'd expect some kind of auto-dock put in place to make sure we land where we're supposed to instead of out yonder by the old tool shed at Farmer Johnson's farm to scare his cattle.
I'm just taking shots in the dark here, just like everyone else, but that's the way I'd plan it.