After playing ME1, and considering it was supposed to be a trilogy from the get go (I'm not even sure if it was public knowledge back then, but let's pretend for now), the logical construction of the trilogy would have been so :
Start (ME1) : What reapers are, what happened to the protheans, what threat faces the galaxy. Subplot : Do you side with the council and champion galactic cooperation or do you push the pro-human agenda.
Ending (ME3) : Considering the revelation of ME1, it's safe to assume the end of the trilogy is about how the galaxy faces the reapers, and how they break the never ending cycle (I assume the most cliché version should have prevailed here). Subplot could be about how this threat is faced by either an harmonious coalition of species or humanity alone, who would have by your actions fucked up the council's power and freed europe from brussels technocrats.
In this regard, I would have assumed the bridge, ME2, to explore the galactic politics further, as it should have been the (more or less impactful) C&C generator for the last part. The actual plot would have logically been : how do you convince everyone that the threat is real and they should prepare. The subplot would be how you either favour the council or mankind in your quest for rallying ressources, allies and support from the galaxy.
Since ME2 turned out to be about how there are reapers out there and they need to be stopped, ie. didn't advance the plot of ME1 one bit but rather stuttered on the same shit, but with poorer delivery (Sovereign in ME1 being presented as giving close to 0 fucks about Shepard while whatsitsname in ME2 being homoerotically obsessed with Shepard, which is mega lame), it's difficult to produce a satisfying ending in ME3, since ME3 has to 'fill the shoes' of ME2 and produce a satisfying result to your choices/the plot at the same time. I have very few memories of ME3, but I remember thinking "well ME2 matters fuck all now, doesn't it ?" and the whole trilogy feels utterly pointless in the end.
Imagine getting a proper ME2, where the threat of the reapers is looming in an even more anxiogenous way, as pretty much nobody in the galaxy gives a fuck about your ramblings and keep on following their petty agenda, some of your fellow human included. This would have been a great theme to explore, and with the basic paragon/renegade system could offer two versions : you Ghandi the galaxy up or you settle for the universal chaos, revel in it and use the reaper threat to hijack political power for your faction.
The missed opportunities and wasted potential of ME2 is excruciating.