Humanity discovered the Mars ruins in 2148. The First Contact War happened in 2157. We got an embassy in 2165 and we got a sit in the coundil in 2183. It's a 35 years span. If you think this is horrible writing, then the whole franchise is flawed for you from the beginning.
We don't know how many time will pass between ME3 and the next ME. If humanity became a council member in 35 years, why other species can't play a major hole in the galaxy in 50, 100, 200... maybe 1000 years, we don't know. Another detail is that humanity got there when things was working normally, now we are talking about a galaxy that just suffered a full scale war.
It's also stated that most other races had to wait at least a century to even be granted an embassy, let alone a council spot. Hell races like the Volus and Elcor still don't have a council spot.
But alas I understand why humans had to be important in the story, I never liked it but I understood it.
I'm talking new races, unheard of races coming in and being major players in the galaxy that I disagree with. Even if the story jumps 1000 years, no new race should come up and overtake the Turian, Asari, Salarian, Human led galaxy. Because if that's the case, the story, it's kind of no longer Mass Effect which is what my biggest concern is. Yes while the new game will be made by BioWare, it's being made at a new studio, with different people who didn't work at all on the original trilogy, they've already decided that they want nothing to do with Shepard, and his crew(well except Liara, she's obviously going to be involved in some way in the next game). The Reapers are gone and unlikely to be heard from again. And if they remove the races we know and create new races to take their spots, it's going to be a new science fiction game with the Mass Effect name tacked on to increase sales. Perhaps I'm overreacting, I like to think I am, but in this era of gaming, nothing is sacred.
Now I'm not saying I'm against new races, if the game truly is about exploration, I expect to run into new races, I just don't want them to be a major player in galactic politics.