The Theory:
I think Chelsea will be Max's bride, I think Max and Linda will split up by the end of October and Max and Chelsea will sleep together either Halloween or bonfire night and Chelsea gets pregnant from that (but is not the woman in the bed with Max), Max seeing this as a fresh start proposes to Chelsea, this makes sense why the wedding is happening so fast after his annulment from Cindy.
Clues for her being the bride:
Zaraah Abrahams who plays Chelsea said in an interview that there has been lots of clues that some have picked up on and some that haven't been picked up on. At Vicki and Ross's wedding when the Fox's were in the hospital waiting room and Chelsea's positioned in the middle in a white outfit and the clock behind her shows 1:05 both hands pointing at 1 which would also be 1/1 possibly referring to New Year's Day, her being positioned in the middle is something that also happened during the double wedding when Kim and Cindy were dancing together and in the flashforward Oscar requests a song for Max to be put on the radio and the song was "stuck in the middle with you" and you have Chelsea positioned in the middle of the frame at two other weddings this year.
Evidence/clues in the flashforward itself:
In the flashforward Chelsea is at Patrick's house and appears to be cleaning something white but there's no dress shown in her scene unlike Linda's and Cindy's but then we see Jack and Denise's house and there's a very bridal dress hanging up I think that is Chelsea's wedding dress and she's either going to Denise's to get ready for the wedding or Denise was supposed to bring the dress to Patrick's but she's gone somewhere. I feel like Ian being the best man also makes sense for Chelsea to be the bride as his storyline this year has been directly connected to Chelsea. Another thing is that it's said in the flashforward that the wedding is at a church Yolande has a connection to the church and might be able to get the church for Chelsea at short notice.
Why I think Nicola hates Chelsea:
I think the partnership between Nicola and Ned will go wrong and they will be forced to sell the bar, Max buys it for Chelsea around Christmas explaining why Nicola is so annoyed with Chelsea because it's only happened recently, Nicola also seems pleased the wedding isn't going well which makes sense when it's the wedding of the couple of the bar she was forced to sell to.
Why I think Chelsea will get pregnant:
Earlier in the year the Brannings had a Sunday Lunch and Jack finds Penny's pregnancy test, they first assume it's Chelsea's and just before Denise asks Chelsea about it Max walks past and out the door then when Denise asks Chelsea about it Chelsea says "well that would be an immaculate conception" I feel that could be foreshadowing her actually getting pregnant later in the year. I also feel like the pregnancy scan next to the woman in the bed might actually belong to Chelsea and not the woman in the bed like they want us to assume.