When a time travelling cover-band frontman steals hits before they're famous, his musical idol descends into a suicidal depression and to save his life he must overcome his lust for fame.

This has serious potential, particularly given if you went down the route of having the idol be an actual historical one... If it was Kurt Cobain it would get a tad weird (although, maybe not...). In a way, I kind of agree with bondthewriter, in that I can't see how overcoming his lust for fame could stop someone from killing themselves. Is it a case where he gives up a stolen hit for the idol to produce?

I'm quite new to this (writing loglines), but here's my stab:

'A time traveling, fame obsessed, cover band singer must learn to give up his dreams in order to save the life of his rock star idol'.

Again, in this logline the conflict feels just internal. Like the MC just needs to get over himself and then the idol will be fine. I;m sure it's not that simple in the story but that's how it feels in the logline. He's ruined someone's life, what does he need to DO to fix it?
I'd figure out a way to get the II back in there, we NEED to know why the story starts.. "Why is today different than the rest?"

This is still too long but maybe something in here will help...
"When a frustrated cover-band musician loses his only gig he jumps back in time to take the spotlight away from his musical idol, who will take his own like unless the time traveler intervenes, giving up his new found fame. "

Thanks.

I debated how to describe the character a lot. He's a narcissist, but it's also important to know that he's in a cover band (specifically). It tells us a lot about the fact that he's lived in the shadow of better musicians his whole life, and it also explains why he already knows the songs and can exploit them when he travels back in time. I just can't seem to trim it down to something that rolls smoothly off the tongue ...

I actually dropped the I.I. after the previous version of the logline. It's after his cover band loses their only regular gig that he decides to use the time travel plot device to make himself famous in the past. I get what you're saying - that the above is phrased so that it appears that the time travel itself is the inciting incident - but the trouble I'm having is that the main character's goal changes at the midpoint. For the first half of the movie (inspired by the I.I. I've described) he's obsessed with the fame and fortune he feels is owed to him. But at the midpoint, when he learns that the cost of his fame is that this musical idol feels creatively bankrupt and wants to kill himself, his goal shifts to wanting to save him.