I'm sure everyone will be happy. Open ended play for those who want it will undoubtedly be available, as will a hard ending for anyone who wants it and can follow some rather simple instructions:

1) Finish main storyline.

2) Watch ending slides.

3) Tap the Escape key to pull up the menu.

4) Select Exit

5) Close the program, or select New Game.

This really is the most logical way to go about it.

Just have the ending slides, and then player gets dropped back into the world as it existed pre-ending..

People like me who like the hard ending will switch off the game, others can continue to play as they wish.

Simple, and everyone's satisified.

That's how New Vegas does it now after one of the later patches. It makes a save just before your sent away to the dam then loads it after the credits.

I'd rather go back to a previous save if I found out I missed a side quest that I got a fail state slide for as well as if a dlc added more slides later for my character's ending.

Good as gold then (apart from not being able to get a couple of unique items, which was a little disappointing).

Pre-ending implies a time warp to a time when the ending is still pending. That would certainly not satisfy those of us who want open ended game play. Alduin has to stay dead after you kill him. Loading a save from before you went to Sovngarde is not post-ending play.

Or, you have the post-ending gameplay so you can switch off the game and never come back to it or start over again, and those that want to continue after the story may continue to do so. You still have a hard ending, and that's you turning off the power button.