Capcom is running a poll in which fans can vote for a character design for a new girl in Megaman Legends 3! They say "new heroine" but it won't be a replacement for Roll.
Here is the link to the English site. Unfortunately, you need to set up a Capcom Unity account to vote!
They also promise that there will be more ways for fans to contribute to the game soon.
Here is a picture of all the females, along with the names of who designed them.
I think I'll have to side with the consensus here and go with Haruki Suetsugu's design. She's currently the leader in votes in the west, while the characters on the bottom row are popular in Japan.
Suetsugu was heavily involved in Zak & Wiki and Zelda: The Minish Cap
And here's an early video of the game.