Should Joker have an ongoing series again? Here's my arguments for yes.
1. He's had one before in the '70s.
2. He's a long-running, iconic character who's a household name, synonymous with Batman
3. Modern TV shows (Dexter, Breaking Bad) have shown us that you can still have an evil character be the protagonist of your narrative.
4. Harley Quinn has a successful ongoing and she's strongly connected to The Joker.
5. You can either emphasize the humor of The Joker's murdering ways like with Deadpool.
6. If you don't like #5 then you can portray The Joker as a man fighting against society and it's rules. (see #3)
7. The Joker is really,really popular and has been for decades.
Whether it's in good taste or not, i'm sure it would sell. There's no Comics Code anymore, the creative team could go 'all out.'