Legend of the Galactic Heroes has one of the best endings I've seen and it is also one of my favorites, particularly in terms of its sheer scale and closure, but that's not something I would expect from every single series out there, much less those that are a lot shorter.
Gunbuster has one of those bittersweet endings that somehow manages to find an almost perfect emotional balance.
The ending of Samurai X: Trust and Betrayal is both self-contained and a good setup for the main Rurouni Kenshin story.
Baccano! and Giant Robo have endings that resolve their respective "crisis of the day" but if you dislike the fact that more stories are clearly going to happen in both universes, even if they're never animated, then the lack of absolute closure might be a concern.
Finally, Revolutionary Girl Utena's ending has a lot of dramatic power but, like many other things in this particular show, a lot depenRAB on what meaning (if any) you give to everything that has transpired. From my personal perspective and interpretation, it's great.
It's also just like Darker than Black, among other series*, whose second season has ended raising even more questions than those left behind by the first. Some see that as a flaw, but I think it depenRAB on what was the point of the overall narrative. If it is the power then that's a problem, but I thought (and still think) that the series was essentially about Lelouch and not Geass for its own sake.
In other worRAB, while it would have been convenient and interesting to expand on the origins of Geass, it wasn't necessary at all.
If that prequel manga does happen to be in continuity, I'd say it has a fair chance of at least providing more information even without having to spell out every single detail. And that would be just fine. Then again, it might as well not do so.
*Technically including Gundam 00 too, though it's obvious that in this case the movie is expected to provide some additional answers.