Casey Hudson returns to BioWare

To say what, exactly? 

Personally I've always thought they would have been better served by an ending similar to the ending to Dragon Age Origins, with elements from the Suicide Mission.

In DA:O's finale, you're fighting your way through a city full of enemies to try to get to the archdemon (a blighted dragon) and kill it. You are special because you're a Grey Warden and if you (or your Grey Warden companion) kill it, you win the war, the enemies flee, and the archdemon stays dead. So you need to be delivered to the center of the city. The entire game, much like ME3, was about gathering allies to your banner so they will fight for you, with some either/or/both scenarios. In this case however, for each circle of the city, you get to choose which allies you want to deploy onto the field. The enemy makeup changes in each section so there are some tactical decisions here too. The more allies you managed to gain, the more choice you have on what you can deploy.

In the Suicide Mission, obviously, you also have assets, though they are single characters rather than groups, like in DA:O and ME3. Again, you get to choose which assets to deploy for what sections or roles and are rewarded or punished for paying attention to their strengths and putting in the extra work. Your mission in this case is to destroy the Collectors - the proto-Reaper comes as a complete surprise - so your actual goal is first rescue, and then sabotage (blow it all up!). 

Both of these scenarios are about delivering something (a Grey Warden, a lot of bombs) to the place they can stop the conflict and win the war.

So, if we applied that thinking, then the Crucible becomes the package that needs delivering to the Citadel. The assets we can deploy are the fleets and land troops that we collected, and any upgrades that we managed to get for them. If the krogans have the upgrade for mounts, then at least some of them may be mounted and more dangerous for it. 

First question to ask is why we're involving the land troops at all, but that's because the Citadel arms are closed. We need to deliver a new package, anyone who can open the arms. It's going to be Shepard, but really it could be anyone who can press a button. The fights on the ground are similar to what we got but we get to choose what units we're deployed with and have some intelligence on what to expect. Those companions/characters who are not with Shepard will presumably join their associated units on the ground, Garrus with the Turians, Grunt with the krogans/his company, Wrex with the krogans, Jack with her biotics. Depending on how well you match up with the foes, some of them may die, I would expect. We saved or gathered their loyalty, we get to deploy them, even if its not actually Shepard making those calls (I don't think it is the Grey Warden in DA:O, it's more of a meta decision). This also gives you the opportunity to try to protect your favourites by not deploying them either with you or in the field. Very characterful. So, you finally get to the controls and open the Citadel arms.

The fleets are trickier, I don't even know what the strengths and weaknesses of the fleets might be, and they're mostly engaged in the same task against the same enemy, getting the Crucible to the Citadel once the arms are open, and defending it until then. I could see some choices here but I'd honestly "play" as Admiral Hackett for this part, or let him do the best he can with the fleets he has available in a cinematic. Perhaps some fleets take heavy losses if you're low on numbers, maybe some odd fleets like the Elcor or Volus fleets have technology that lets them act as force multipliers for the effort (refueling, massive shields, repair). Whatever fleets end up on the vanguard are likely to get chewed up. Either way, the fleet manages to get the Crucible to the Citadel, with or without crippling casualties, and it links up, and goes off.

And it evens the odds. Maybe it drops all the Reaper's shields or fries them half to death, leaving them crippled. Maybe it seeks out and destroys the AI in control of them which just so happens to reside in the Citadel, though it looks like an AI Reaper in this cinematic obviously, leaving them unable to communicate with each other and disorientated. Whatever happens, it suddenly makes the conventional method viable. Now, whatever fleets are left go to town, destroying Reapers as they gain the upper hand. Perhaps some escape if the fleets were devastated and have to be chased down later, meaning they lay waste in the interim. Perhaps instead they try to destroy the Crucible and the Citadel with it, with you on it, but a healthy fleet manages to intercept, meaning Shepard survives.

Then, after the cinematics wind up, a funeral, or a party, with possibly missing members of your little family, the option to talk to them (like in DA:O and DA:I) before you go somewhere, to receive a medal or something from the Council, I don't know. I'd also have the memorial wall at the party. Obviously this party, depending on your choices, could be lively or rather... sparse.

And then finally the traditional slideshow for every faction and character that you care about. This might go on for a while, if a lot of them are still alive! 

Not artistic enough  

The Borg in Star Trek were also introduced as an unstoppable force in TNG. A single cube was able to destroy an entire fleet and then, at the end of Voyager, a simple virus and the Voyager, a single ship, more or less obliterated them.

This is good, but really not that far off from what we got. You're still using a super weapon to do some space magic against the space Cthulhu's.

Why not just have Shepard learn that destroying Harbinger would cause the reapers to stop the harvest?

Because AIs are awesome.

So awesome, in fact, that our puny organic minds cannot fathom the depth of their awesomeness.  We need to be merged with them in order to become worthy of sharing in the awesome that is synthetic life.

The same could be said about dumb dumb when it says it preserves  life in reaper form yet the reapers are put in harms way

Then you don't understand Art and simply don't get it.  

It is inevitable, now that we know it is possible

Yeah, the reapas are pretty dumb.

But mah endings!!!

Actually, that sounds pretty cool. It is one thing I like about the DA series, though, truth be told, I don't have a dog in the fight about the ME3 endings. Itr didn't really bother me and I'm not so invested in them that I'm willing to argue for them.