I think one of the best things they can learn from the ME trilogy is to provide us with an enemy that is a powerful threat but not being built up as unstoppable gods.
I knew about 30 minutes into Mass Effect 1 that we were going to need some last minute space magic to beat the Reapers. They were writing themselves into a corner from the very start of the series with that.
Personally I would also prefer not having one big drastic choice at the end of it all but rather have the outcome be a culmination of everything I've done up to that point. I would say sort of like how The Witcher 3 did it, but in reality the ending you got was based on a handful of conversation choices with Ciri.
Personally i'm going to wait and see what the Andromeda gets in reviews and if Bioware dose not at least explain the ending of ME3 in the Andromeda game I will most likely not buy Andromeda. Some may say thats steep or a narrow sighted decision but, I was burned once and if Bioware will not give some closure or explanation to the trainwreck that was the ending of ME3 then there is no reason for me to trust their game making in Andromeda. Don't get me wrong I want to like Andromeda, but fool me once once shame on you fool me twice shame on me.
No amount of explaining it is going to make it suck any less.
The only fix for the ME3 end would be to rewrite it completely.