Having finally beaten the game... I was pretty okay with the ending, honestly. Not perfect, but it sure as hell wasn't disappointing. But my personal, 'impossibly good' version (of the Destroy ending) would likely go as such:
-More questioning of the Child, though I did note that it itself pointed out that the cycle was no longer an option. When it was created, it was a viable one, but with all that happened in the events of Mass Effect, it's not any more.
-A way to simply destroy the Reapers. Or... hell, I'm even fine with saying that it only targets Reaper-like intelligence/synthetic signatures. Which, for all we know, EDI and the geth have advanced beyond, since it's shown they're also capable of compassion, forgiveness, etc that the Reapers don't seem to have. ... I couldn't kill the synthetics, damn it (I really did enjoy the synthesis option, but for my 'ideal' ending, destruction without hurting EDI or the geth is necessary).
-I would have preferred the Normandy/crew remain on Earth, so having them there. Quarian liveships would remove at least a fair bit of the whole 'what would they all eat' issue, and since the geth were already helping the quarians re-adapt to the homeworld, they could certainly help with adapting to/rebuilding Earth. I can also see massive-scale terraforming of Mars, too, just to give everyone some extra space- and who knows, maybe there, there's schematics on the Conduit (I honestly can't remember if the archives were destroyed back in the beginning of the game. But for these purposes, they're not), which helps lead to re-creating the Relay system.
-Make it clear that the Relay explosions would not destroy their systems, as the energy was turned into something compeltely different, all that would happen was some space debris.
-Krogan babies, maybe Mordin-the-girl-krogan working in a lab and humming something. Rachni, asari, geth and krogan (due to skills and lifespans) on stations as they rebuild the Relays. Some small follow up on the individual characters- personally, I don't have many ideas on what it'd be, aside from like... Liara's father (I can never remember how to spell her name) getting a long-overdue apology for her suggestion to build their own Relays, and leading the first push to rebuild them. Miranda and Oriana leading a refugee camp for all species. A turian on Omega (which, as it always has, has gone back to being a violent hellhole) praying to the spirit of Archangel, to give him (or her) the strength to defend the innocent of the station. Eventually, Liara having a spot on the Council, leading to...
-However many centuries or even thousands of years later: The galaxy is as peaceful as it'll ever be- the geth are still as helpful as ever and have a joint embassy with the quarians on the new Citadel, the majority of krogan are still cool, etc. But there might be krogan splinter groups trying to invade everything might be a problem, the batarians might be completely losing their crap from slowly going extinct, I dunno.
But for one day, all that seems to cease, and all attention is focused on Kajhe. Shepard was found alive in the rubble, but she was still incredibly injured, and lived only three months after the end of the war. As decreed in her will, she was cremated and her ashes have been preserved until interstellar travel was once again possible. If, by everyone busting their chops and working ceaselessly, it's only a thousand or so years later, Counselor (and by now, Matriarch) T'Soni giving a eulogy for her (if it's so much later... why not Zoidberg EDI?), before releasing the ashes into Encompassing.
And because they deserved their happy ending... somewhere in the galaxy, a human Spectre and her drell boyfriend raise a toast as they watch a live feed of the presentation, before going back to whatever target they've been pursuing.
... yeah, even if she lived, my Shep would never have had a nice little 'reunite with LI' ending. So I'll go with the thought of her dying a legend, but the various reincarnation/wheel of life thing being right, and allowing her and Thane another chance.
And man, that turned out cheesy. ._.
