I often wanted to check out dominions 1 to see how the previous game played. But since I already had dom2, the improved version, I wasn´t interested in paying for an older version.
So here is my suggestion: Add dominions 2 for free to dominions 3. That way you will get even more positive word of mouth( Two games for the price of one!) and it won´t really cost you anything, unless there is a significant amount of people who still buy the older version after a new version comes out. Hey you could even add dom1 so I finally get the chance to check it out

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So how many people bought dom1 after dom2 came out? If it´s an insignificant number, I think this would be a good marketing move.