Rob Van Dam is currently scheduled to return to WWE on April 7th, the night after WrestleMania 30. It's expected that upon his return, he'll have another several-month run before likely taking time off again.
RVD returned to WWE at the Money In The Bank pay-per-view last July. He took time off following his match with Alberto Del Rio at Battleground in October, but it's been expected that he would return at some point.
The latest regarding Sting's debut and future work with WWE is that he'll only be wrestling one singles match with the company. Of course, the expectation is that if he's only working one match, it will be against The Undertaker at WrestleMania 31 next year. However, it's unknown whether or not WWE will decide to reveal those plans on WrestleMania Sunday or the following night on RAW.
It is worth noting that Sting is in New Orleans during WrestleMania weekend. It was announced that he'll be doing a special, and very rare appearance at the annual WrestleCon convention taking place that weekend in New Orleans.
Sting has recently stated publicly that he would like Taker to be his final match. However, this is the first report I've seen where that potential match would be his first AND only.