Here are a few loose ends that need to be addressed in the final season of How I Met Your Mother.
The mid-season finale of Season 7, titled "Symphony of Illumination", introduced us to Robin and Barney's future kids, only to find out by the end of the episode that the kids were imaginary and that Robin is infertile. Her infertility.lead to the breakup between her and Kevin, a season later. Ted, Lily and Marshall all know about Robin's infertility. The only person who doesn't know (apparently) is Barney.
It seems weird that Robin told Kevin about her infertility the moment he proposed, but hasn't told Barney about it in the five months they have been engaged. Maybe she is afraid the Barney will reject her when he finds out about this, or maybe she hasn't told him because she knows he is not that enthusiastic about kids after the false alarm they had in the winter of 2011. Whatever the reason be, this issue has to be addressed before Barney and Robin get married.
It is also possible that Barney already knows somehow, and Robin doesn't realize that, or maybe Robin already told Barney offscreen. The latter, however, seems highly unlikely, as they would not hide something like this from the viewers.
Robin's bullfighting career
At the end of same episode mentioned above, Future Ted mentions that Robin was "briefly a bullfighter". We have not seen this taking place in the timeline of the show, and is probably something which Robin does after getting married to Barney.
Ted and Victoria's breakup (v2.0)
After getting back together after 7 years, when Victoria left her fiancKlaus at the altar for him, Victoria and Ted broke up again when he chose his friendship with Robin over his future with Victoria. When Lily said that Robin would be touched when she hears about this, Ted asked her not to tell her. At that point, we hear Future Ted saying that Robin did find out about it. So we can be positive that this will be something discussed in the final season.
When of the huge mysteries of the show is the origins of The Pineapple, which Ted found on his nightstand after he woke up in bed with Trudy, in 2005. Although Future Ted that they never found out about the pineapple's origins, viewers are hopeful they will be told about it eventually.
Throughout the show, Barney has never acknowledged what he actually does for a living. We know that he worked for AltruCell Corporation, and then later Goliath National Bank after AltruCell acquires GNB in a hostile takeover. At one point a member of Sven addressed Barney as "Head of the Search Committee" at GNB, but even after that Barney denied telling his job to the gang. This is not really a loose end per se, but we would like to know it.