That is, add to common settings:
$wgDefaultUserOptions['watchdefault'] = 1;
At least on Wikimedia projects, I think it makes sense to assume that a new user who registers and edits a page wants to follow subsequent edits to that page, rather than to create a bunch of new pages and follow only those: most of our new users make only an edit o two. If the user knows how the watchlist works and don't want it, they'll just uncheck the box when editing, but as it is they'll most likely lose sight of any following edit.
This will be particularly useful when T30026: Enable e-mail notifications for watchlist (EnotifWatchlist) on all wikis is completely fixed.
T47020: Make preferences "Add pages I create and files I upload to my watchlist" and "pages and files I edit" true by default
T40796: Set "E-mail me when a page or file on my watchlist is changed" (enotifwatchlistpages) to true by default for new Wikimedia users