Is your ‘scan’ fully aligned and merged?
I’m assuming your scan is in mesh form and ready for conversion to ‘surfaces’. So, at that point there’s of course many approaches you could take but the most enjoyable workflow imo is mentioned here:
and:
The approach I prefer is the upmost automatic as possible, though one doesn’t necessarily exist 100% yet.
As far as ‘sketch plane’ or what not, I’m uncertain if you mean simply just the over all orientation of the object in the workspace or not.
Scanned objects do pretty much end up having a somewhat random orientation over all at some point. And yes this needs to be resolve by the user in some way.
I used to use a piece of software extensively that basically had some sort of ‘bounding box’ type approach that the user could use to find the basic outer most extents by which to square up the objects to the work space. While there’s several ways to do this, it’s mostly up to the user to decide and track this particular transformation.
Here’s a cool image that illustrates a permutation of a workflow example from right to left:
I think it’s a good example for what’s possible cause my mesh was so bad and had holes everywhere. 
I better go get to work at my dayjob though, but this is one of my fav subjects 
bbl 