I think there is pros and cons to this. Long story short, I'd probably pick no, that I am content with the three days. Let me elaborate.
Some of the high points would be that there would be more days for the NPC characters to interact and have places to be. We would get to see even more characterization in arguably the most developed NPCs in the series. That part I would immensely enjoy. I would hope that if we had seven days instead of 3 we would also be exposed to more characters and perhaps slightly more extensive/larger areas to explore. The sidequests could be more abundant or more extensive as well. Now, that last sentence is also one of the cons.
I think the downside to this idea is the extent to which the dungeons and sidequests would of had to be extended. Part of the allure of MM was the difficult and the time restriction. It is meant to be challenging. Complete each dungeon in under 3 days. Now if you were given those exact same dungeons. but seven days. It would be rare I think that you would ever have to reset time. Therefore, if the game were changed to be a seven day cycle the dungeons would have to be extended...by a lot. That is more than double the time we were originally given. This applies to sidequests as well. The Anju Kafei quest beign the first to come to mind. You would need more events spread out over more time. So if we were to double not only each dungeon and each side quest, plus add extra characters, areas, and new side quests sounds fantastic at first glance (because we will have so much more to explore and play), but it is also like redoing the entire game is it not?
Does redoing the entire game cause it to loose its heart and the essence of the game that we all love? MM is one of the games that in my opinion has the most soul and is the most put together and whole. When changes like this are proposed I think it is important to think about what the heart of the game is centred around, in MM the time clock and the three days is a very large part of that.