I know I'm echoing the sentiments of others by saying that I want the combat to be like mount and blades.
Honestly, I probably won't be 100% satisfied if the combat isn't in the same style as mount and blade. That doesn't mean I won't enjoy the combat greatly if it's different from mount and blade. I just feel that the combat in mount and blade is the best all around. It's simple, but leaves plenty of room for mastery.
I know all the videos we've seen are pre-alpha, but the combat that has been shown is incredibly stiff and clunky. I look forward to testing the combat and other features to help polish them as much as possible.
With all of this being said, I'm expecting combat similar to fencing. Sorry for getting slightly off-topic.
I've never had a crafting system I was satisfied with so I think SBS really has to start from scratch there. It won't be complex enough otherwise.
With building I think they will be fine as long as players are able to place blueprints that can be seen in 'build mode'. The same way stuff is built in ARK or any other survival sandbox game with the exception that the structure isn't actually built instantly. Perhaps leaving behind just a frame. Then taking resources and the parts required from blacksmiths and carpenters and assembling them on-site.
EDIT: OH! The construction system in space engineers is a pretty solid example. It's actually quite nice and I highly recommend taking a look at it.
Alternatively, if players are able to download a plot of land into an architecture tool, then create the blueprint there, and then upload it back into the game and paste the blueprint over said plot of land, that would work as well... I just feel it would probably be too much data bouncing around.
As I said previously though. I look forward to testing during the alpha phases. I don't really want to say that SBS should 100% use the systems from another game, because they may produce something even better, and I think it would be a shame to deny them the opportunity to do so.