I personally don't want to see Shepard ever again. Shepard spent the majority of ME3 bellyaching and acting like a child. I would like not to revisit that. It's okay to start fresh and whatever they decide to make canon after the events of ME3 is what it is. There is no salvaging it so I'm moving on.
Whatever they come out with for the new Mass Effect series will probably sell like hot cakes. The gameplay is top-notch and fun, which is the bottom line when it comes to gaming so people are going to play it. If the campaign doesn't work out, there's always the multiplayer, whether you like it or not. It has given ME3 tremendous replay value, for my part. I still play it because of the diversity of characters. It's fun and I look forward to where they take it in the future.
But I'm also still interested in a new story in the realms of the Mass Effet Universe. If they open it up to choosing between what species your main character is in campaign, it will add a new element that we haven't been able to experience. I would be all over that. As it is now, mulitplayer is my only opportunity to play as something other than human, and Bioware has created such a diverse Universe - it would be a shame not to utilize it.