Depending on implementation, I probably would have taken it over Star Child. It certainly didn't help in ME3's case that the ending/Reaper motivation essentially rehashed the Organic-Synthetic Conflict, which was at best a sub-theme of the series.
At least it played into a theme of some degree, one that indeed has been present in the series from the beginning.
I certainly would not have taken dark energy over the Catalyst and OvS. One may be a rehash, but the other involves fundamental plot holes and broken, circular logic that put everything in the shipped ending to shame.
Even better, I would have preferred to see the Reapers as essentially trying to prevent the use of WMD's, from ever increasing technologically advanced civilizations. The culling could have been their method from stopping anyone from reaching a point where they could cause galaxy-wide destruction.
That still makes them inarguable "good guys" and Shepard their antagonist, while also implementing some unusual and archaic pro-weapon themes. "No, seriously, we're different! Trust us! Let us live and keep our guns!" Not to mention further questions about why civilization wasn't cut down sooner, before previous devastating wars.
Something along those general life-preserving lines is how I envisioned the Reapers' motivation, though, as soon as Sovereign started talking about order-vs.-chaos and advanced technology. The shipped ending just took it a step further with the creation of AI instead of advanced weaponry. It's still the same roundabout premise: self-destructive technology that develops beyond civilization's control, only that's a lot more interesting and philosophical than WMDs. Though, frankly, anything beats the Reapers' insistence on using the relay technology under the dark energy concept.