Anything below stated is for survival mode, I realize that creative mode may or may not have these issues:
Important UI option = Text size must be adjustable SOMEHOW PLEASE!! In the Java version, the lower you set the resolution, the larger text becomes, I have a 1920x1080 monitor, but play in fullscreen at 1152x864 specifically for this reason, otherwise I can't read the tiny text of some things. Don't hold back on this, the players should decide if a single letter or sentence takes up 0.1% of the screen or 100%. Players should be able to decide if the text and UI is small sleek and out of the way because they can read just fine, or if it's big, intrusive and in your face because they can't read as well due to age, disability, genes, etc. Readability is what kills modern games. Ark Survival Evolved is a perfect example, teeny tiny text on a blue on blue on blue UI, ever since it was implemented in 2017, I stopped playing, I just can't handle it. Don't make this mistake, please! Give us power over text and UI scaling pretty please! Auto scaling based on resolution is great, but manually adjustable values (maybe in a config file?) are necessary as well since not everybody's eyes are the same and "perfect scaling" may be perfect for most, but not all, manual adjustability without limits is the best counter to this and will enable 100% of your players to have perfect readability instead of 60% or 70% only.
Bulldozer/Wrecking Ball (or a better suggestion that has a similar function) = Even with the Steel Sledgehammer, tearing down entire buildings with hundreds of blocks is obnoxious. These vehicles will make it painless and quick, just tear through it! Wonder how much of a performance impact this would have.
Steamroller (or a better suggestion that has a similar function) = Flattening masses of ground takes so long, 10 square meters is fine, but what about 100 or 1000? This would rectify this. Somehow choose a starting point, kind of like a base for the terrain height, wherever you start the Steamroller could work, and every small hill and bump you roll over will be flattened to the height of the starting point. Only lowering terrain, not raising it, for this, something else would be necessary.
Lawnmower = I heard this suggested a year or two ago by somebody and fully support this as a perfect way to get rid of grass, obsoleting the scythe completely.
Aircraft = Anything from a simple Hot Air Balloon (actually sounds awesome) to single engine planes like Cessnas. A seaplane like the Seawind 300C would also be great as it can use water for takeoff and landing as well as the ground! Maybe obnoxious, but a freaking Antonov AN-225 Mriya or any generic cargo plane as a whole would allow us to transport the entire freaking map in one go, haha! This is just wishful thinking though as cargo planes are just plane awesome. Something like the Douglas C-47 Skytrain would also work as it is a smaller cargo plane but still can haul a lot.
Submarine/Scuba gear = There is plenty of potential underwater, I heard scuba gear mentioned at one point but never subs. I am fine with either, but since we're eventually gonna have cars, trains, electricity, etc. why not a sub?
Gemstones & Resources = Ruby, Sapphire, Emerald, Topaz, Diamonds, these can be useful aesthetically as decorations and prized possessions, to show off like rich people do with art and antiques, or red51 has in his mansion, and also for building, plenty of things use Diamonds due to their obnoxious strength. What about the return of the blue ore, Cobalt? If we can think of anything to use it for besides blocks. What about certain metals that are only found in hell? Certain strong and desirable ones like Titanium for example, give us an important reason to dig to hell instead of just because it's awesome, I mean, it IS awesome, but the more reasons, the better! Crystals could be found in certain caves at lower depths as well, varying colors. Could be purely cosmetic reasons, or maybe an alternative to sand specifically for creating glass. Probably some technological purposes in real life too. I would certainly love to place/hang some crystals in my territory just because they look beautiful! Underground oil wells, this could be a cheap yet powerful way of powering modern technology like vehicles, generators, etc. these could be the first/lower tiered vehicles, whereas higher tiered ones could use some more advanced materials, just like in real life. I will assume that there will be at least one train variant that is powered by coal, a steam engine train? Oh, how about a steam boat! YEAH!
Some antique vehicular weaponry, i.e. Basilisks, Culverins, Trebuchets. Maybe allow the bandits to build and use these as well to tear down your buildings in an attempt to siege your land? This would obviously be an optional server setting, disabled by default, but why not? At this rate why not just take Empire Earth 1 from 2001 for inspiration? 
Permanent "Invite to group" feature = This should be self-explanatory. I have one friend on my dedicated server, every time we logon, we need to be physically next to each other to be invited to the same group, so we can see each other on the map, see a red bracket in the world locating each other, etc. Would be nice if similarly to the server.properties admin setting, we could have something like this for a group to eliminate the annoyance of always having to re-add each other every time we logon. I consider this a low priority of course as it is a mere annoyance that doesn't ruin anything at all.
Stack size setting = I don't know about you, but I would certainly like a somewhat consistent stack size. Torches being 32 and Wooden torchmounts being 64 seems just silly. A setting to define a stack size for an object would be nice so that an issue like the aforementioned does not happen. 64 being the max stack size in general is totally acceptable though, no complaints here! While on topic, why does the Big smelting furnace only have room for 60 ores when a stack size for ores is 64? Couldn't we just have two rows of 32 slots instead of 30 slots in the Big smelting furnace?
Do note that anything noted here is not specific but rather meant as an example. If anybody, has better suggestions that do the same thing, i.e. a better suited, rising world-esque styled, way to flatten ground en masse instead of the Steamroller, suggest away! Same goes for anything else noted here, I have the ideas, but not the vehicles or tools that would make sense. I have suggested way too much now, I'll shut up, haha.
A "high" spawnrate is indeed quite problematic, since NPCs always spawn permanently, and over time, a dungeon may become overpopulated
