They need to make an ending in which the Enterprise and her entire crew are lost. Everyone is getting sick of these jolly good happy endings that Star Trek seems to portray all the time. I mean, in Voyager Janeway got her crew home safe and sound, brought home technologies from 30 years in the future and, oh yes, they pretty much destroyed the Borg.
In TNG, all the Enterprises of the 3 time periods crash together, and poof, everything is back to normal. The crew is safe and sound, and the ED has not a scratch on her. In DS9, the Dominion is defeated and everyone lives happily ever after.
The endings are getting very boring. I think in Enterprise the crew should have to make a choice of destroying themselves in the process of saving Earth or something like that. That way it will not be a completely happy ending, but it wouldn't be too depressing either =)