While Rockstar may have GTA Online covered DLC-wise for April, what lies beyond is still a mystery. Following this month's one-a-week lowrider release schedule, some players feel that after the Sabre GT, there should be a break from updates to Benny's Original Motorworks — but this doesn't mean that vehicle customization should be shelved as well. There have been plenty of DLC ideas floating around fan discussion boards which would give Rockstar plenty to jump off from. But what if they've already have things planned out?
A player recently uncovered (most likely through a trainer) some hidden modifications to the LSPD Vapid Cruiser. Neither the police nor civil variants can be owned by players or taken to Los Santos Customs, however the GTA V game files prove that the vehicle was at least intended to be customized.
While we've known of similar hidden modifications for other vehicles — including planes, the paint-jobs of which could have been altered — this one is new, which possibly means Rockstar has been sneaking in features of a future DLC well ahead of release to dodge data-miners and leaks.

While a Biker DLC for GTA Online is by far the most hotly requested, the massive success of the LSPDFR mod proves that there would certainly be an audience for police-related content. Of course, news of such a DLC would be pretty massive, which could be why Rockstar planned ahead and spread out pre-loads across months of other DLCs.
Then again, we might just be looking at cut content, however the customization range seems complete enough, so Rockstar really had no reason to cut it. A new corrupt cop/undercover cop update with new contact missions for a clandestine organisation within the LSPD (think HR from Person of Interest) would sure as hell satisfy players calling out for a more significant update.