Thank you very much for your kind words, I'm happy to hear that you like the game 
Firstly, a simple change that i'd like to suggest is a higher reward rate in stone mining
Probably this isn't a bad idea. Well, one of the next updates introduce higher tier tools (which speed up the mining process). Once they're available, we'll tweak the whole mining process (and adjust the mining speed as well as the reward rate) 
Secondly, even though there is a lot of building blocks in the game already, I would love to see even more. Perhaps even more variants of cobblestone, stonebricks, and perhaps also a half-timber style block
Actually it is our intention to introduce more textures, however, unfortunately this has an impact on the overall performance (but there is still some room for some additional textures). Originally we also wanted to rework and replace some of the existing textures, but obviously this would cause a lot of trouble for existing worlds (so we won't touch existing textures until the game has more extensive support for texture packs [e.g. serverside texture packs]).
Blocks with roof shingle textures of different types would be really useful.
That's true, roof textures are really missing. However, in order to avoid having flat roof textures, we first want to wait until parallax mapping is implemented. If this provides satisfying results for roof textures, they will be added, otherwise there will be probably a special roof construction element or something like that.
Thirdly, I know it's been discussed before in one of my recent posts, but I think that the worlds should have a larger playable area
The world is indeed infinite (at least the world generation isn't restricted), but you will run into precision issues (rounding errors) sooner or later. These issues become noticeable at around 10,000 and are getting worse the further you travel (somewhere between 10,000 and 100,000 it becomes unplayable), so it's recommendable to stay within the 10,000x10,000 boundaries.
Unfortunately I have no ETA when the "real" infinite world will be finally available ( @DaBoiye: I just used my 'ETA' hotkey again :D), first we really need to get some other stuff ready (especially the new player models) 
Also, it would be one of the greatest games ever made if you could implement technology like train building tools, cars, planes, boats, airplanes etc. OMG that would be the coolest thing ever.
At least trains, cars and boats/shipts are planned It's likely that there will be also some basic aircrafts at some point (e.g. a glider, and/or a hot air balloon). Unfortunately I can't say much about planes or other aircrafts (like helicopters), maybe they will be added once the game leaves early access
Ok, another suggestion I have, and this is the one I want the most, is rivers. Please, I beg you, we need rivers!
I'd love to add rivers, unfortunately it's quite tricky to add proper rivers to the bumpy terrain. First we need to get dynamic water ready. Actually it's also our intention to rework the terrain generation in the future, maybe rivers will be part of these changes.
Also rideable horses would be epic.
Rideable horses (as well as camels and donkeys) are in the works. They will be added after the player models update
And lastly, will there ever be real time shadows in the game? Hopefully that wouldn't be too hard on the graphics.
Real time shadows are definitely planned. They will have an impact on the performance of course, but there's anyway some room for optimizations. I'm pretty sure that shadows will greatly improve the overall appearance of the game^^
Do you still run into the same issue? In this case please try to verify the integrity of the game cache: 
It's difficult to say what caused the "unknown error" while uploading the file, usually trying it again should do the trick (or are you still unable to upload the file?)
We added a 2 seconds "cooldown" though, but since we also synced the blueprint placement with the server (in order to have a cancellable PlayerPlaceBlueprintEvent for the API), this caused an unintended 10 seconds delay... This will be fixed with the next update 
I have to admit the name of this function may be misleading. Actually it is supposed to return the timestamp of the date when the player was previously online (i.e. if I play on a server on March 1st, then take a break and join the server on April 10th again, this method will return the timestamp of the 1st of March).
