If a synthesis ending is chosen in ME3, I would first like to see the removal of the glowing green eyes, and circuit-board skin... on clothing? There's gotta be a better way.
Second, I would like to see it take place several hundred years after ME3. That way all the galaxy repair has taken place and things are flourishing again. And the novelty has worn off all the "hippy love-in" stuff. The geth and quarians have figured out how to live with one another on Rannoch, and are co-operating in studying dark energy. The salarians have continued to be salarians, and their scientific research and knowledge gain is rising near exponentially, and being used for all sorts of things, financial gain, political alliance... mainly just power.
Tuchanka is much further along the road to restoration, and the krogan population has re-stabilized after the war. The synthesis effect has lengthened the time between prenancies, and lowered the number of krogan born at a time, normalizing their reproduction rate somewhat. Under Wrex and Bakara's rule the clans are now all Urdnot, though individual clan traditions are still observed. Bakara's teachings and wisdom has become a religion, and her followers spread her teachings to anyone who will listen. But Tuchanka is still becoming crowded.
The turians maintain a strong military force, because it's in their blood, it's what they do. Duty and Honor have taken on a greater signifance for the turians. It's how they rebuilt their home - and their armies - so quickly. In the beginning of the reaper war aftermath, it's how they cope with the reduced numbers of their population, which in turn caused them to withdraw to their home system, and setting up "customs" blockades around all the relays leading there. The turians have become xenophobes.
The asari had also withdrawn to Thessia to try and rebuild, and although they have begun to venture out into the galaxy again, they're still struggling with finding an identity in the new galactic community. Interacting with them is rare, or at least uncommon outside of, say the Citadel or a planet like Ilium.
The other species have gone back to being themselves too; The Volus restored the galaxy's economic system in record time and have taken the asari seat on the Council. The turians maintain a seat, but their councilor is dismissive of most everybody, and increases tensions wherever he goes, whoever he talks to. The salarians maintain a seat, and now are viewed as the peacemakers, mostly because of the influence they have.
But the humans are unofficial leaders of the council, their voice to the galactic community. The citadel has remained in orbit around Earth, and humanity is the dominant species in the galaxy. The memory of Shepard still commands that much respect. But the initial population decrease from the war and the synthesis effect of increasing lifespan has reduced the human galactic population significantly, but not nearly so much as other species. Military strength has also decreased, and diplomacy and trade have become better strengths for the humans.
The reapers, from Harby to husks, and the surviving Leviathans, have set off into deep space exploration, sending back regular reports of progress through deep space. it's been an uneventful 500 or so years.
And there you have it. From that I'd like to see tensions rise over initial disputes about resources. Eezo still has to be found. Maybe skirmishes on Eezo mining planets develop into major battles from time to time. Maybe the turians destroy a diplomacy ship on its way to Palaven over some code violation. Tensions rise. Maybe the salarians are getting close to some scientific breakthrough that will enable a major play for political power.
Anyway I'm kinda rambling here, but I'd love to see something like that for the next game, if it must be synthesis.