I would like to point out that the current ending of ME3 is a buzz kill because basicly all the work you do in ME1&2 and ME3(up untill the ending) is basicly a waste 1. you die 2. either one of the three choices results in the descruction of the mass relays and the citidel (except control option, the citidel isnt destroyed) and 3. the normandy crashes on a uninhabited planet or a moon.
I think that either choice will probably throw the whole galaxy in the chaos as the mass relay are destroyed, which the entire galaxy relies on for traveling, and will result in all the fleets that are part of the sword and sheild fleets (the asari, turians, geth etc..) being stuck at earth because without the relays they have no way the get home.
Also that 20 second cutscene at the end of the credits could be taken as a reference to another installment, as when you look at the sky there are two moons (or a plant and a moon) and their silhouettes are similar to the ones that are visable when the normandy crashes. Also the dialogue of it also could refer to another installment: "tell us another story of the Shepard","Its getting late but ok". But the current ending/s prevent another installment.
These are just my views of the story