In the recent sixth season finale, it emerged that Barney (Neil Patrick Harris) was getting married at the mysterious wedding. However, it was not clear who he is planning to tie the knot with.
Thomas has now revealed that viewers will get more information when the show returns in the fall.
"The premiere of season seven starts off at the same future wedding where we learn a little bit more, though I can't say exactly what we learn because that one doesn't air until September," he said. "We just learned one half of the equation in the finale, that it's Barney, and we don't know who the other person is, but in season seven we'll - I can't say exactly when or how - but we will obviously learn the other half of that equation in season seven."
Thomas added that Ted (Josh Radnor) will be "out there dating" again in the new episodes and explained that several instalments have already been planned.
"I think November sweeps of season seven, which we've already planned out, I think each episode is going to end with something utterly mind-blowing, so I'm really excited," he said. "That's one of the things about getting later in the series run - you can start calling some bigger plays and moving the larger story, the larger arc, down the field and at a faster rate, and it's exciting.
"I will say that there is some very, very interesting Barney and Robin stuff coming up for the first half of season seven, but I don't want to say anything more than that."
Thomas has previously claimed that the seventh season is a "blockbuster" with "nuclear-powered stuff".
How I Met Your Mother was renewed for two more seasons in March and will continue to air on Monday nights when it returns in the fall.