I've had classes where people were asked (in a rubric) to reply to at least 5 classmate's posts, in addition to writing something original, and that is what they did, but the work (judging as a fellow student in the class) was mediocre. A better rubric would have been to leave the amount of posts out (something more than 1 is implied), but specify the content more and what is appropriate/good content for the response.
I've also had classmates, by the way, that went way above and beyond the call and really provided thoughtful insight to most posts (very demanding I think, but I learned from them)
I have no problem with bringing in game design. But I do agree grinding (doing menial tasks in order to level) is not engaging and does not necessarily promote learning.
The open ended portion works with motivated students. I think for open ended we do have to look at the learner. If it is the online introductory college accounting course, students may need more encouragement to participate. The students may have never taken an online or a college course. So some guidelines in this type of introductory course can assist students in learning how to participate as an online student. We use a rubric in our basic accounting courses and also what we we call evolving scenarios. The evolving scenarios have the basic setup at the beginning of the week, a plot twist in the middle, a second turn of events and then a wrap-up at the end of the week. But they are required to do original posts and then respond to classmates or the instructors' questions. So what John Fritz's presented on students being able to compare themselves to others in the class may very well be another way other than rubrics to let students learn the habits and culture of being a better student.