As tempting as it may be, I would say do not cover Janis Joplin unless she can do it justice.

Connection - Elastica
I Am The Highway, Like a Stone - Audioslave
Heaven Beside You - Alice in Chains
Rock Lobster - B52s
More Than a Feeling - Boston
Rhiannon - Fleetwood Mac
Walking After You, Everlong - Foo Fighters
Sweet Child O' Mine, November Rain, Don't Cry - GNR
Good Times Bad Times, Babe I'm Gonna Leave You, Stairway to Heaven, Going to California - Led Zeppelin
All Right Now - Free
Why Can't I - Liz Phair (I don't care what you say... that song is seriously catchy... a guilty pleasure)
Hey Hey My My, Keep on Rockin' the Free World - Neil Young
Man Who Sold the World - David Bowie/Nirvana
Another One Bites the Dust, Killer Queen, Save Me, Somebody to Love, I Want to Break Free - Queen
My Favourite Mistake - Sheryl Crow (a cool groovy bluesy type song)
In the Meantime - Spacehog
I Saw Her Standing There - The Beatles
Roxanne, Can't Stand Losing You, So Lonely, Message in a Bottle, Every Little Thing She Does Is Magic - The Police
Fall to Pieces - Velvet Revolver
Even Better Than The Real Thing, Gone, Do You Feel Loved, All I Want Is You, I Still Haven't Found What I'm Looking For, One Tree Hill, Bad, Sunday Bloody Sunday, New Year's Day, Two Hearts Beat As One - U2

Maybe some Bon Jovi, Soundgarden, STP?

Obviously some of the above songs would need to be transposed to fit her vocal range, but these are basically some songs that I would think "Oh, wicked!" if I heard them broken out by a female-fronted rock band.