10. Original Trilogy Cameos
Depending on who you talk to, some people are clamouring for ME 3 to not be the final resting place for their favourite respective characters - whilst others think a clean break is what's needed instead. Regardless, thanks to having such diverse racial traits both aesthetically and culturally, there's a very good chance that certain established character/racial traits would carry over anyway, like any Krogan's tribal roots, a Quarian's spiritual eastern traits, human's position as the 'newcomers' to the galaxy etc. This essentially opens up the possibility for a new squad to have a variety of things in common with the original crew either through action or dialogue, but in the end it might be better or more assuring to simply drop in actual crew members depending on where 4 takes place in the timeline.