I hear you. I'm trying to offer that a sequel could come up with something. It will surely be part of the fun though. What could possibly top the reapers?

How do you come up with an enemy that is more fearsome than the reapers in a prequel? Nothing is as threatening as the reapers in the past, but something could potentially be invented that is more threatening than the reapers in a sequel.

I vote sequel all the way.

Here are some problems I had with a prequel:

1) Any prequel is a retread that we already know the ending to. It would be playing a game knowing all the spoilers.

2) Choice will limited. You won't be able to affect the outcome if you play a war such as the First Contact war. Mass Effect is about choice. If you choices don't affect the outcome of the wars and conflicts you take part in how do you possibly hope to have a Mass Effect game? Mass Effect has such an extensive lore on all the topics people bring up as possible storylines that it would be pretty hard to create even small-scale choices that you would actually be able to affect. Choice is made meaningless.

3) As a follow-up to problem #2; if Bioware elects to do a story that is concurrent and avoids any of the major struggles or stories that took place in the previous trilogy and also avoids wars and stories from the lore then why on earth make a prequel? The point of prequels is that you see what built up to what has already happened. So to make a completely new story and set it as a prequel is redundant.

4) Looks like prequels may have us play as a previously established character like Garrus or Javik. I love those guys but playing as them? I couldn't really have any choice because I know everything they did already. I'd have to be playing to make choices that I think THEY would make not what I would want MY character to make. Plus, I know the endings of their stories also! Playing as Garrus or Grunt or whoever only makes sense if you get to play with them in a sequel so that you can determine what choices they make and they will actually matter.

5) It's cheap. It's unoriginal. I think bioware can do better.

Reason why a sequel is better:

1) Our choices made in the trilogy can be reflected. Why did many dislike the ending? Their choices weren't reflected in the final mission! Why on earth would we not want to see the impact of the decisions we made at the end of the 3rd game? If you thought the ending was so bad that you don't want to see what happens after than why on earth would you want to see what happens before? Even if Bioware picks a Canon ending (which I hope they do not) we can at least play a game in a universe where Shepard's actions and the actions of his crew meant something. If they actually make the game reflect all 3 endings as possibilities then I don't see how you could not find that intriuguing. It will be like buying three games in one! Why throw out our save games?

2) A fresh start is what these games are all about! I want new choices. I want a new universe to explore. I want a new character to play. I want to a new crew to meet and interact with. I didn't pick up Mass Effect 1 because I was like "oh okay I can't wait to play the stuff I read in that codex entry". I played it because I wanted to play as a character who could affect the universe and characters by his choices. It was exciting, unpredictable, unique, and above all; original.

3) We can get new material with a few familiar faces. Liara can live a long time. So can some of the other characters. I'm sure they can show up in this new game as cameos or even crew members. Wouldn't it be interesting to see how they could react to your new character in a post-trilogy universe? Basically, you don't necessarily lose all the cool characters.

4) New bad guys who won't be pointless. Nobody who you will possibly fight in a prequel could be that intimidating because they will always play catchup to the Reapers. There was nothing as dangerous as the Reapers running around in the galaxy when they were present. Now you will have a chance to encounter a new enemy that could potentially rival the threat of the reapers from the original trilogy. Even if they don't at least you will get to fight them without knowing they never did significant enough damage anyways. If you do a prequel, you can't possibly create a threat equal to or greater than the reapers because... why did we never hear about them? Did the reapers know that whatever this threat that existed was is entirely capable of defeating them as well? I want new bad guys that are actually worth taking down, who will do unpredictable things, and with the reapers gone it frees that up. With the reapers still around any bad guys will be playing for second place.

5) Technology of the world will be able to be more advanced. New guns. New armor. New advances in tech all around because it won't take place in the past.

6) New government systems. The alliance has always been in charge throughout all the history pretty much. A prequel will retread the same galactic politics. Boring. I want a game that allows room to have the politics of the universe be up for a change. 

7) I chose control so I totally want to see how Shepard being in control of the reapers pans out. It could be amazing to even have a conversation with Shepard in reaper form as a new character. Especially if that Shepard in Reaper form reflects all the choices I made when I played as him. On the other hand, how cool would it be to see the grave of Shepard, meet Shepard himself (if he lived in the destroy ending), or if everyone is synthetic-organic hybrids? I want to see how those endings play out. They are too epic to pass up!

8) NEW RACES! New races can show up and become new players in the universe. We already know all the races from the past. This allows new horizons!

9) Speaking of new horizons; other galaxies could possibly factor in. We could encounter a race from another galaxy and perhaps we can use their methods of transportation to go and visit their homeworlds? In a prequel, nothing you discover will be of any importance because all the discoveries were already made. I'd rather make discoveries that are actually discoveries. I'd rather learn new things about the universe that have not been known about already and that can potentially change the universe as it is.