Already I've seen internet rumors of a possible Mass Effect 4. However before people start taking this as fact we need to remember a few things.
1. Bioware has already stated that Mass Effect 3 is the conclusion of Shepard's story. They further backed this up by saying that the endings would diverge into wildly different conclusions because they didn't need to continue the story. ( That was the whole reason people were angry about the endings )
2. Many of the people who believe there will be a Mass Effect 4 usually talk about what happened with Halo. However Halo is a first person shooter and the ending of Halo 3 left room open to continue Master Chief's story. Bungie didn't write themselves into a corner unlike Bioware. Instead Bungie ended Halo 3 on a cliffhanger that could be used however the next developers wanted.
3. Many people who thought the Mass Effect 3 endings went against the fiction of the Mass Effect universe felt that Bioware had lied to them. Since Bioware has already said that Mass Effect 3 is the conclusion of Shepard's story more fans would again think that Bioware is now just trying to milk the franchise. ( With EA's help of course )
However this doesn't mean there will be no more Mass Effect games. Although Halo 4 is using Master Chief it is in itself about a completely different story. Although the core elements of a Halo game are still there it is no longer about The Covenant and the flood but rather about The Forunners and fighting a new enemy. ( Look up the E3 gameplay to understand ) If another Mass Effect game was made it wouldn't be about Commander Shepard but most likely about another part of the game universe, rachni wars, Metacom War, Krogan Rebellions, The Morning War, or possibly something in the Mass Effect universe before the Mass Relay's were discovered.
1. Rachni Wars: A game about the Rachni Wars could be along the lines of a first person shooter. Maybe going through the Rachni caves to clear them out and getting lost resulting in a survival horror game.
2. Metacom War: Possibly a third person action game with limited choices. More of an action adventure game than an RPG. The game would focus on spec ops like raids before major confrontations. The more you damage the enemies infastructure the easier it is to take your objectives during the actual battle. Could possibly include space battles and the ability to control vehicles.
3. Krogan Rebellions: A massive strategy war game with hundreds of units on the field at once. Something like the Total War games. Space Battles, taking or destroying whole planets, gathering intelligence, etc. This game would be all about huge strategic battles.
4.The Morning War: A game were you get to choose one of two charcters. A geth who is simply trying to survive and defend itself from the Quarians or a young Quarian female ( somewhat like Tali ) who joins up with the Geth to do what she believes is right. The game would heavily explore the Quarians culture before they were kicked off their planet.
5. The Mass Effect Earth before discovering mass effect technology: There was most likely different conflicts on Earth and it would be cool to explore them. Since this would take place before mass effect technology it would open a whole bunch of stories up that although set in the Mass Effect universe would have a completely different atmosphere.
Please leave your ideas.