I will be happy to list the things that I would like to see both in general goals and some specifics that may indicate the things that I'd like to see.
In general the goals that I want the ending(s) to support are:
-- A multitude of divergent endings where choices you've made in the series to date have a direct impact on the outcomes of Shepard, his squad, and galactic civilization.
-- I want to have more freedom to write my own story so the choices available in the endings need to support this freedom / variety
-- I want the endings to make sense based on the rules defined by the Mass Effect Universe up until the point that Shepard nearly gets hit by the beam weapon in London.
-- I want an epilogue that shows the fate of Shepard, his squad, and each of the major galactic civilizations in the short term and in the long term. I want to clearly understand how Shepard's choices effect the universe.
-- I would like the ending(s) and epilogue to promote a sense of closure and not rely on earlier parts of the game for this sense of closure.
Ideas on how to support the goals listed above / speific examples / etc. :
-- Adopt the "dream / hallucination / indoctrination" theory. Basically the in-game endings that we see now could be left largely intact. With enough EMS and a "destroy" selection Shepard seems to wake up [or at least not be dead] Have Shepard get up and finish the fight.
-- Keep what happens before Shep is nearly hit with the Reaper beam weapon intact. If the footage where he sees his squad "killed" is restored I certainly wouldn't complain

-- I would think this should be avoidable in at least some playthroughs.
-- Examples of elements within the divergent endings could be:
>> Allow Shepard to die from Harbinger's beam weapon. The Reaper wins, the galaxy burns, the cycle continues.
>> Shepard dies but inspires his squad to activate the Cruicible and stop the Reapers
>> Shepard suicides because the Reapers are hacking his implants or have almost fully indoctrinated him .. he does this so his squadmates can activate the cruicible.
>> Shepard assumes the mantle of the Illusive Man
>> Shepard lets the Illusive Man live because his original mission is still important
>> Shepard sacrifices himself for the good of humankind to activate the Cruicible
>> Shepard sacrifices himself for the good of the galaxy at large to activate the Cruicible
>> Shepard succsessfully activates the Cruicible and it works as hoped. Shep lives and stays a soldier with the Alliance / Council spectre / whatever
>> The above but Shepard tries to find the simple life with the LI of his choice
-- In terms of Epilogues if Shepard dies lets have a funeral complete with eulogies and similar.
-- If Shep lives then show what he's up to in 5 years and maybe 20 years. Having vidmails, news reports that Shep is watching, whatever could give us an idea how his squad and the galaxy is doing.
-- For the Tali fans maybe have a full render of her and not just a picture if you're a LI
-- If nothing else an epilogue slide show like Dragon Age: Origins could be used to give us an epilogue.
-- Having someone find one of Liara's tributes to Shepard in 50,000 or 100,000 years may be a cool element esp. if we get opinions of what they think of the find.
-- Restore some of the deleted content if useful as I found the CUT VERSION of Anderson's & Shepard's conversation to be quite powerful and it spoke to me much more than the ingame version. This would perhaps be a good time to add in a dialog choice prior to the RGB endings so we could pivot between which conversations to use here.
Feel free to message me if you want more details