On April 27th 2018 ABBA announced they have joined forces again: 

"The decision to go ahead with the exciting ABBA avatar tour project had
an unexpected consequence. We all four felt that, after some 35 years,
it should be fun to join forces again and go into the recording studio. So
we did. And it was like time had stood still and that we only had been
away on a short holiday. An extremely joyful experience!
It resulted in two new songs and on of them "I Still Have Faith In You"
will be performed by out digital selves in a TV special produced by NBC
and the BBC aimed for broadcasting in December.
We may have come of age, but the song is new. And it feels good."

The two-hour special entitled ABBA: Thank You for the Music, An All-Star Tribute will feature virtual Agnetha Fältskog, Björn Ulvaeus, Benny Andersson and Anni-Frid Lyngstad performing their brand-new song “I Still Have Faith In You”. 

The final trailer for Mamma Mia! Here We Go Again has seen the light of day, watch it below.

About the movie:
Ten years after MAMMA MIA! THE MOVIE, you are invited to return to magical Greek island of Kalokairi in an all-new original musical based on the songs of ABBA, With the film's original cast expected to return and and new additions joining them, MAMMA MIA: HERE WE GO AGAIN! will delight audiences when it opens on July 20, 2018. 

With a soundtrack full of ABBA songs not featured in the first film - along with some reprised favorites - MAMMA MIA: HERE WE GO AGAIN! once more brings the producers of the original fil, Littlestar's Jusy Craymer and Playtone's Gary Goetzman, together. Craymer is also the creator and producer of the smash-hit stage musical. Ol Parker, writer of The Best Exotic Marigold Hotel, writes and directs. Benny Andersson and Björn Ulvaeus are aboard again to provide music and lyrics and serve as executive producers.