there are a lot of placeholders in the game yet as it is still in development. I am sure the dev will consider more different stone-types in the future. I agree with you it would be nice to have marble and others needed to create the blocks. however for now i am fine with it as it is.
better land generation - yes, taht is definitely in planning. probably could be released already but it held back because it would break existing worlds. as there will be several new things that can / will cause a world break these things get released together, to keep the frustration of starting anew at a minimum.
if you look at the effort and time people put into their worlds this decsision can only be supported - even though its itching me, just as you probably, to see a more realistic world creation.
combat mechanics - not sure what you mean with improvments here. RW isn´t a fighting type game and probably will never be. the API will make it possible though to add more weapons, tools, armor and models in the future. HP, DPS; etc. i am sure can also be adjusted then.
originally the land above was supposed to stay free of magic in order to keep the realism-feeling. the underground and hell get the magic touch. for me this is fine, though magic was often requested, so a kind of fantasy biome is considered. also there the API will allow mods that can give the magic feeling as well.
about the art style i have to disagree strongly. i am very glad RW is giving us the freedom to decide on our own what we want to do and how. i like to mix things. if you dont want modern - just dont build it. the crafting-stations will get reworked as well, so maybe the order in which you can build what level of items (old - new) will be adjusted.
building is not what is currently getting worked on. not sure what you mean with your remark. survival is getting fleshed out, as well as means of transportation (horses already introduced, boats as well - now trains are in the pipline).