Originally posted by Science Dog:

To piggyback on this, we don't even know what those mobs will be at this point, so who knows if the base defenses that we all love (walls, moats, etc.) will even be effective against them.

To interject here...
"We" do not all love or need base defenses (walls, moats, etc) ;P
https://steamcommunity.com/sharedfiles/filedetails/?id=2424562622
To be clear, some players have built marvellous outposts with walls, moats, etc. just as some players have built wonders without any defenses, but not all and in any case defenses are not necessary.

As for building within Mistlands or any other as yet undeveloped biome such as Ashlands or Northlands, do as you like, but be prepared to have to rebuild once they are implemented.
I have no concerns at all about saying goodbye to my little homes from home in Mistlands, Ashlands and Northlands (none of which have walls, moats, etc. ;)
And as for the new mobs, the game has always provided you with the commodities and tools to deal with them too, so I expect no change there either!