There are a lot of content that is leaked. First of all, new VIP missions and challenges and new in-game phone number were discovered. This could possibly mean a new in-game contact. Apart from VIP challenges, new adversary modes were discovered. The names are Inch By Inch and In And Out.
You can read the forum posts by the guys who found out the leaks below.
Here is TezFunz’s post about VIP challenges:
And this link leads to a post by Yan2295 who uncovered new adversary modes:
According to forum posts, the leaked content isn’t going to be released in April. Probably, Rockstar is planning to released it in May/June.