I recently went to a test screening of Fantastic 4: First steps and I'm going to share the ending on here.
The battle between the Fantastic 4 and Galactus reaches a point where Sue Storm and the Silver Surfer are in a 1v1 and they both have a lengthy conversation whilst fighting [/spoiler
[Spoiler] The conversation is a deep take on femininity, motherhood etc. Silver surfer recounts her time as Shalla-Bal and Norrin Radd and how she was pregnant (like sue was )before galactus made her into the silver surfer and this makes Silver Surfer join the Fantastic 4's side after empathizing with Sue [spoiler/]
Silver Surfer pulls a magic cube out of her surfboard and tells the team that using it will destroy Galactus but also this universe. The team is left with no other choice than to use Reed's multiversal ship to escape. Silver Surfer flies directly into galactus and there is a massive explosion that cuts to black. Her fate is unknown
The Fantastic 4 ship was damaged in the process and is arriving in the main marvel earth without a way to multiversal travel
Post credits reveals that Silver Surfer survived and made it to the MCU, Dr Doom doesn't appear at all in the movie or is even mentioned or hinted at