As mentioned in today's review, the X-Factor team was originally going to be Cyclops, Beast, Angel, Iceman, and - since Jean Grey was dead - Dazzler. In fact, all the promo art for the series tried to play up the mystery of who the fifth member was going to be:
And if you look at the last panel of Dazzler #42 (final issue in the series), it has Beast basically inviting her to join his new team:
But when word started circulating about the plan for X-Factor, John Byrne told writer Bob Layton about an idea Kurt Busiek had for a way they could actually get Jean back without needing to forgive her for murdering an entire solar system. And if you've got a choice between "the original five X-Men reunite" or "four of the original five X-Men reunite, but also Dazzler is there," of course you're going to pick the first one. So Dazzler was out, Marvel Girl was in, and the team as we know it was formed.
It's also worth mentioning that what we think of as the first X-Factor costumes are really just the first published X-Factor costumes. As you see in the promo art up above, they were wearing different suits, with the shape in the center looking more like an hourglass than a strongly defined X. With just a few weeks left before the book needed to be sent to the printers, Jim Shooter decided it wasn't good enough and made the team redo the entire thing; much like another comic that had to be done on short notice, the production was aided by the arrival of a timely storm (in this case, Hurricane Gloria, at the end of September 1985), and the book was finished mere hours before the deadline. For whatever reason, the costumes were redesigned during this marathon session, becoming the big X's we know today.
And finally, while the final cover was drawn by Walt Simonson, Layton had submitted multiple versions that all got rejected. One of them is seen up above; here are two more, also showing the old-style costumes:
Cyclops is hugging her and she's surrounded by fire, how hard could it be to guess who she really was?