As long as the canon ending mirrored all my decisions - most notably the states of the Krogan, Geth and Quarians - it would be okay; although having the Reapers "just disappear" probably wouldn't work for Synthesis or a Control Shep who would take a hands-on approach. To avoid disappointment they'd have to put in an awful lot of work creating multiple starting states or set it so far in the future the galaxy would be unrecognisable.
Personally I would prefer a prequel, the Mass Effect universe is an interesting mish-mash of some of my favourite sci-fi and space operas held together with a liberal sprinkling of space-magic and I'd like to explore it some more.