So, everyone has their guesses on what could possibly happen in the season finale as well as the series finale. I have 4 guesses:
Guess 1. (this was posted as a comment before) Maybe it would be something like, when Barney proposes to Quinn, Quinn accepts. And in the flash forward, It is seen that Quinn is the bride. In Season 8, (if Becki Newton stays longer which is very possible.) we would see more about Quinn and Barney as an engaged couple, but as things are happening Barney realizes that he can't hide the fact that he's not over Robin. He struggles to forget about Robin and Quinn (without him noticing) notices how Barney has been struggling. So he calls Ted, as the best man, for some advice. Ted gives him some mushy advice that will help Quinn realize the right thing to do. So as the ceremony is going on, Quinn backs out, tells Barney that Robin is the right girl for him and they end up married.
Guess 2. It could also be, Quinn does not accept the proposal. Then in the flashback, it's seen that Quinn is the bride. Quinn is off the grid for some time. In the middle of Season 8, Barney accidentally bumps into Quinn. They still have unresolved feelings for each other so they go out to talk. When the two talk to find closure, they find themselves stuck in the moment, kiss and Barney proposes again. This time Quinn accepts. They tell the gang. Everybody is happy (including Robin) and they get married.
Guess 3. My 3rd guess is, when Barney proposes, Quinn accepts and in the flashforward, Robin will be revealed as the bride. Like what I said in Guess 1. Barney will struggle for his feelings with Robin. Quinn will notice this. Few weeks before their wedding, Quinn puts Barney to the test. When the test is over Quinn will find out the truth and talk to Barney. She tells Barney, who would be completely indenial, that he still have feelings for Robin and suggests that he proposes to Robin. He does what Quinn says. He proposes to Robin and Robin denies at first thinking that it's wrong because she knows how Barney loved Quinn. Barney reasons with Robin and she finally accepts. On their wedding day, Robin talks to Ted, saying if she did the wrong thing because she answered Barney right away while he was recovering from his breakup with Quinn. Ted tells her to follow her heart. She calls Quinn, and Quinn comes to the wedding. At the ceremony, Robin then says that she loves Barney very much and she wants him to be happy. So she gives him to Quinn, saying that Quinn is the perfect girl for Barney and they end up getting married.
Guess 4. Last guess. Quinn does not accept the proposal at first. Barney, disappointed, goes to Maclaren's. While drinking alone at Maclaren's, he sees Robin come in. Robin also notice him and goes and talks to him. Barney then tells Robin that he proposed to Quinn and that she didn't accept. Robin feeling bad for Barney, talks to Quinn about her not accepting Barney's proposal. She tells Quinn that for a smart girl, she was stupid for not accept Barney's proposal. She tells Quinn how amazing Barney is and how she's going to miss out a lot (hinting that Robin really still have feelings for Barney). She leaves and this makes Quinn realize that Robin was right and meets Barney. When the two meet, she accepts the proposal. Throughout season 8, we will see more of Quinn and Barney. We will also see that Robin is not handling it too good, but she does not let anyone see it cause she's afraid she's going to screw everything up again. As the wedding drew nearer, pressure on Robin started to snap and she finally tells Lily. Lily, being known to not keep secrets for too long, tells Ted and Marshall. The three swear not to tell Barney, ever. A few days later, Barney and Quinn have started to give out invitations. Robin decides not to go the wedding. Barney then hears about this and talks to Robin. Robin then tells Barney that she's still inlove with Barney and the only reason why she talked to Quinn is because she knows that Barney deserves someone better than her. This makes Barney realize that he still loves Robin but pushes those feelings away. On their wedding day, he was having second thoughts and calls Ted for advice (as seen on "Big Days"). Then Quinn called Ted (as the best man) to ask how Barney was doing. Ted says Barney are having second thoughts. Quinn asks why and Ted says it's nothing. Quinn then asks if it has anything to do with Robin and it leaves Ted speechless. During the ceremony, when Quinn is asked if she wants to take Barney as his husband, she says she can't because she knows that Barney is inlove with another girl. Barney denies this until Quinn says that she knows who that girl is. Barney says that if Quinn leaves him, he'll have no one cause Robin didn't even come to the wedding. Then, Robin comes in and Quinn says that he will always have Robin with him and she called her to come. Robin and Barney gets married and Ted finally meets the mother.
What do you guys think? These are only guesses. Feel free to comment. :)