if you want to continue ME, without make 3 diferent games for each ending, you can start the game 100 years after the reaper war, if the player choose control or synthesis, you can say in the codex that there is an advance specie who came from another galaxy (andromeda maybe) that have even more technology than reapers and they are looking for minerals or eezo and the milky way, it´s not a war like in ME3 with all species involved, just the council or the citadel species, if after the reaper war there are new species in the council like krogans if you cure the genophage, in that war the reapers get destroyed (if you want, you can make a game about that war) the species of the council win that war and keep their eez, and adquire new and advance technology that give them the habily to travel to the andromeda galaxy and find new species, if you don´t want to create to much species you can say that that specie annihilate a lot of species in that galaxy and there only remains a few species that gave them their eezo... to deal with the destroy ending, you can see just a few quarians or geth if you don´t make peace between them, and just a few geths if you choose the destroy ending...
elements I want to see in the next Mass Effect:
1.-more RPG elements like exploration like in ME1, land in a planet, explore with a vehicle and sometimes get out of the vehicle and enter in some caves with dangerous animals, or in a merch headquarter,etc... also you can give the players options to join a mercenary group like blue suns, eclipse, talon if they allow humans to join, you can join the alliance or another group if you create more. joining a merch group or alliance,be a thief, a vaul thunteretc... will give you some missions to do in some planets like (if you are from alliance) you have to go to a planet where a merch group have red sands and stop their plans, or protect a colony from the attack of the blood pack,etc..(if your are from a mercenary group) you can do missions like kill a military general, destroy a military base,steal weapons, (if you are a vault hunter) you can talk to every mercenary group or even the council to take some mission to earn some money. if you join any of these groups you also obtain some exclusive guns,armors and powers.
it will be nice if you can have a improved relationship with some npc´s that want to join your crew or leave it if the start hating you, make that things happen talking to them or taking some decisions they don´t agree with you, and you can kill those npc´s or something else.
2.-keep dialogue wheel, less auto-dialogue and make that dialogue wheel can give you more control of the situation like if someone it´s going to give you a mission you can say yes, no, try to get a better reward if you complete that mission, obtain information about the mission (you can say yes or not in the paragon or renegade way)
3- keep the paragon renegade system.
4-keep the mass effect 3 combat system and improve it if you can, make than spacebar/X/A button don´t do everything :S
and add some new features like sneak behind the enemy or hide, if you are a full paragon you can obtain some extra points if you don´t kill innocent people o you only kill your primary target, or that new system can be useful in recoon missions O:
5- add some species in the milky way like the raloi (I really want to see them) and protheans (you can say that some ships found a planet with proteans, the have not espacial technology because they don´t want to face the Reapers again, and then they realize than Reapers have been defeated or controled,etc... and then join the galactic comunity, if you want with the same or diferent society :S
6.add more arquitecture,vehicles, weapons,armors and clothes to the species, to give them even more personality

7-the story: you can use the story that I told you, or just pieces of it to the next games (but I have told you that story to deal with the endings from ME3. but I think it´s a valid option (a advanced super advanced organic race looking some resources or eezo in the milky way because their system it´s almost empty of those elements.
8-add some personality to uncharted or colonized planets, I mean not just strange animals, you can also add different flora, like Ilos, actually in ME1,ME2 and ME3 a lot of planets looks like if you were still in Earth, so variate flora only if you can to give more personality to each planet.