I really like this idea and I've put a lot of thought into how something like this could work, I'd love to discuss it further though and I'm really glad you've made a thread about it because this is a huge thing Blockland's missing.
There are two aspects to sharing saves which I'm interested in exploring - the first being what you've suggested which is the sharing of completed save files but then I'm also quite interested in a place for people to share "props" for builds which I'll explain a bit later on.
I've put a lot of work into a system for hosting save files on RTB already, there's a lot of backend code in place for it to work without taking up a load of space on my server which was a bit of a mission. At the moment it also works out what colorset you
should download in order for the save to look correct, as well as what add-ons you'll need in terms of custom bricks, vehicles, lights or items etc. and lists them alongside the save file too.
Unlike the add-on submission system there'd obviously be no requirement for approval so we could actually have a true ranking system based on user ratings (which isn't exactly the case with the add-on system at the moment) to enable popular high quality builds to appear at the top. I'm also having a think about the idea of using tags for builds instead of categories since I feel it'd be far too hard to sort all of the builds people make in Blockland into 15 or 20 categories. People will definitely need more ways to categorize and describe their work.
As for the props, by this I mean small items people have made for decoration such as street lights, couches, kitchen appliances or any of that stuff which most people aren't necessarily good at making but would like to have to add the finishing touches to the house they've built. I'd probably put a limit on the size of props to be like 8x8x8 or 10x10x10 just to keep them reasonably sized, too. They'd function in much the same way as full save files except I think these could be categorized a bit easier into groups like furniture, machinery, city infrastructure etc.
The only issue I've got with props (and builds actually) is how to load them into the game. In an ideal situation you'd be able to hit the "load" button on the in-game save browser and it'd equip your player with a special brick which would be a square with the exact dimensions of the build you're trying to load to make it a bit easier to position and everything. Obviously that's not possible at the moment but I might pitch the idea to Badspot and see how keen he is on adding some kind of system to do that. If anyone else has got some ideas about how to place builds you're loading when you're not exactly sure about it's dimensions then definitely discuss them in this thread.
Anyway I think I've written enough to get a discussion started here.
Ephialtes promised this a while back, but has not yet implemented it
It's a big job to do it effectively and to do it right - I don't intend to rush it by any means. I've promised it and I'll deliver it like all the other features of RTB I've promised and delivered.