Personally I like the PI on the citadel thing. Taking characters who we've already seen and know what happens to them, well you can't make any danger in the game seem real. You know how it ends, it's like reading the last five chapters in a book and then going back to the begining. It's not nearly as satisfying.
One thing I really want in any future Mass Effect is greater character customisation, more races, more classes not just the same old Fighter/Mage/Thief setup we got here and more options for characters, something like the feats system in D&D or the Perks system in fallout. Giving us more options will allow us to create a character that is truly unique.
"There are many parts of my youth that I'm not proud of. There were loose threads - untidy parts of me that I would like to remove. But when I pulled on one of those threads, it unraveled the tapestry of my life."