1. Mass Effect 4 should be a sequel, although a spin-off game as a prequel would be nice.
2. Mass Effect 4 should have Shepard as a supporting character or a name which gets mentioned various times throughout the game and could have a major plot line.
3. Saved files from previous games should be able to import over to Mass Effect 4 somehow, although it may be practically impossible if Mass Effect 4 is on next-gen consoles only.
4. New Villains. If the Reapers came back directly in Mass Effect 4, it will be to repetitive in my opinion. The series should have a fresh villain which isn't quite as powerful as the Reapers and not planning on having a galactic war perhaps.
5. Choose your own race
6. Space Battles would be brilliant if done in the right way in Mass Effect 4. Being able to command your own ship and fight against another in a very detailed combat system has always been one of the things I feel missing from the series.
I'll give you a rough idea of a plot I made a few months ago:
It's 2193. The Mass Relays and the Citadel were partially rebuilt after the activation of the Crucible 6 years ago by Commander Shepard within 1-6 years (depending on your choice from Mass Effect 3 and your EMS.) All of Galactic Society was at peace following the Reaper Invasion and united and rebuilt the Mass Relays and the Citadel together.
If you chose Synthesis in ME3 some races aren't comfortable with their synthetic parts and Galactic Society overall finds it controversial.
If the ending was control and you were renegade Shepard the Reapers controlled by Shepard watch planets mysteriously as if inspecting them. Eventually Renegade Shepard's mind makes the Reapers colonize other planets (although most planets are inhabited apart from the Drell Homeworld which is now overpopulated.) It causes suspicion from the Council but finds it bearable.
If the ending was Destroy and you had low-average EMS score races are at war with each other for scarce resources. Only the Asari, Humans, Turians, Hanar and Elcor aren't at war and are trying to make peace just like before. If Wrex wasn't killed at any time in the events of the previous three games the Krogan have a seat on the Council for the first time ever and have expanded to more worlds and built colonies on inhabited worlds. The Krogan assist the five other races above for galactic peace if Wrex wasn't killed and Grunt is Wrex's Assistant or Second in command if you like. However if Wrex was killed the Krogan are expanding on inhabited worlds just like the Krogan Rebellions and are at war with the Salarians and the Batarians. Although the Council Races have made a deal with the Krogan to not colonize any of their worlds, which the Krogan don't for the time being. Regardless of whether Wrex survived or not, the Salarians are at war with the Krogan as they see the Krogan overpopulating the whole Milky Way and are kicked out of the Council due to their brash choice.
If the ending was Destroy but you had the Perfect ending Shepard is found on an unknown planet alive but barely breathing. It takes nearly 6 years for Shepard to be back on his feet following the Crucible's activation. Depending on the circumstances in ME3 between Shepard and your Love Interest Shepard has a kid.
More to come of the plot.
Are there any ideas you would like to share?