I'm not exactly thrilled about the 1.17 1.18 split, but I have been trying to find a silver lining- (Besides the obvious 'when given more time, the new features will likely be better and more thoroughly tested.) One thing I came up with was that this split provides an opportunity for the developers to better integrate the new and upcoming features with existing features.
...That sounds neat, but to be clear on what I mean by this: One thing that I think developers should always be careful of when updating is not just the introduction and balance of new features, but how they can be used to enhance a variety of aspects of the game. Mojang has historically done a pretty good job with this- there are very few mechanics and parts of Minecraft that don't have a good reason to exist. (Ahem... bats.) Increasingly, their strategy seems to be to enhance and improve upon existing content- this is a very intelligent development plan on several accounts- firstly in that it ensures the game is gradually tending towards a more cohesive product; and secondly because the prospect of negative feedback is far less when you're improving things you know players already like, rather than adding things that are utterly new.
Mojang generally finds a perfect balance of this- improving old systems with new features and eccentricities. The Nether Update, Village Update, and Update Aquatic have all been great examples of this.
I'm not trying to dissect the development process here- in fact, this is getting a little overreaching for the main point- that any new additions don't merely need to fill some sort of niches or have useful mechanics; but that they need to be integrated with the existing features. Consider it a 'welcome the new while honoring the old' sort of philosophy. However, there aren't really that many examples of that- largely since most of the new features have been in areas where existing features weren't present. Before the Village, nether, and Aquatic updates, the villages, nether, and oceans were pretty bland.
The Caves and Cliffs update is different primarily because it is one of the first updates to make some really fundamental changes to the Overworld, as well as enhancing outdated systems. The 1.17 update is going to add a lot of the features themselves, while the 1.18 update is supposed to bring terrain generation changes- essentially, 1.17 will add features, and 1.18 will implement them. And a lot of these features seem to be a lot newer than anything Mojang has tried for awhile.
It's a rather roundabout way of putting it, but I think 1.17 gives an opportunity to add minor examples of implementation for features that will be more extensively added in 1.18. Partly because some of these features seem a bit too exclusive to new areas of the game. Ideally, there shouldn't be a palpable divide between new and old. And you can sense this in some parts of the game- such as how barren the End is compared to other parts of the game- (This could be put off as an aesthetic choice, but it has been some time since the End received a considerable update, or how basic many of the structures seem compared to new structures.
There's been a lot of improvement in this regard- such as the recent tweaks to mineshafts. However, some of the new features seem in need of balancing, or seem a little highly specialized.
-Moss Blocks are a cool addition with unique properties, but are apparently going to be exclusive to the moss caverns. In 1.17, they could be implemented in other areas- such as bits of moss in kelp forests, or jungles.
-Rooted dirt is another block that seems needlessly specific towards Azalea trees. Rooted dirt could generate in single blocks beneath other trees, making dirt a bit trickier to remove in forests. (And also provides a way of adding rooted dirt in 1.17.)
-I'm not sure whether or not copper is going to be implemented, but it currently doesn't have that many uses, and quite a lot have been suggested. The new ore veins are pretty interesting, but, (unlike other metals), it doesn't seem like Copper shows up elsewhere in the world- unlike Gold and Iron. Gold is found in Ocean Monuments and Bastions, and Iron can be found in the form of bars and doors and chains and stuff. Considering how common copper is, it doesn't seem too far fetched that it could be integrated into villages or other structures. (How this might be done could become clearer as we get more applications for copper.)
-Amethyst is a cool and unexpected addition- and perhaps small pillars of amethyst could be found in deeper caverns? Or witch huts or something?
-Candles would also look cool in witch huts, villages, and wooded mansions.
-There's a lot of ways deepslate could be implemented- namely, I've heard it mentioned numerous times that the strongholds ought to be made of Deepslate now that they're to be found deeper underground. Other structures- such as Illager Outposts, Wooded Mansions, and dungeons- would also look good with a bit of deepslate blocks.
-Other ideas about minor implementations of 1.18 features? I'd appreciate feedback!