Would you consider as an improvement, if it would be possible to fail the main quest, not by picking another side, but by not being able to succeed the task you were set to.

FoNV, for instance, already allowed you to pick a side and make that side win. So if you choose to support the Legion, the NCR will fail, but the side you chose would win.

It was possible to fail being able to support the NCR all the way, by failing to save the president. But doing so doesn't mean you lose, because it is still possible to pick another side.

Would you have enjoyed to support the NCR all the way, but fail at the very end, because you are unable to save Oliver from an attack from Lanius, or because the Legion assassins successfully overunn the dam, as you didn't kill enough of them while rushing for Lanius ?

So, basically, you would still win the game, but fail the main quest. Instead of being praised, you would be depicted during the slideshows as amongs the NCR runaway, and your involvement forgotten or judged with shame.

Doesn't mean that the last chapter of the main quest would be overly hard, but would the additionnal possibility of failue add more stake into the final quest, and maybe better replayability and you would want to try out all win/lose scenario for all factions ?

This applies to Fo4 but might be interesting for other games as well...