In the CBS sitcom's sixth season finale, viewers saw womanising Barney (Neil Patrick Harris) prepare to marry a mystery woman in a flash-forward scene.
"We will meet who Barney is marrying in season seven," Thomas told IGN. "We're not going to string that along."
He added: "[The identity of the bride is] the big central question, though there are many, many new questions."
Co-showrunner Bays also claimed that viewers should "absolutely not" assume that the bride is either Barney's current love interest Nora (Nazanin Boniadi) or ex-girlfriend Robin (Cobie Smulders).
"Don't assume we are beyond or above doing a little bit of Mamma Mia in season seven," he joked. "There will be some [other] choices."
Back in March, How I Met Your Mother was picked up for both a seventh and eighth season.
"This year we'll be figuring out if and how we'd go beyond that," explained Bays. "If there's more story to tell and people would keep watching the show, we'd totally be open to doing more."
How I Met Your Mother returns to CBS on September 19.
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Watch a clip from How I Met Your Mother's sixth season finale below: