Hypothetically, an active cellular watch would be fine paired to an iPad. But when you get down to it, there would be issues. You still need the parent iPhone app for most Watch apps and many iPhone apps aren’t available on iPad. And at its core, the Watch is still meant as a compliment to the iPhone, not a replacement. The addition of cellular hasn’t changed that.
Hypothetically, an active cellular watch would be fine paired to an iPad. But when you get down to it, there would be issues. You still need the parent iPhone app for most Watch apps and many iPhone apps aren’t available on iPad. And at its core, the Watch is still meant as a compliment to the iPhone, not a replacement. The addition of cellular hasn’t changed that.