Now, coming off of WI Super Mario Bros. (1985) had a female protagonist? by Drunkrobot. What if Sega had made Sonic a girl?
It would be extremely dangerous to type 'Mario and Sonic' into Google Images (and if my POV was still in effect, then 'Maria and Sonic' would be even worse).
In all seriousness, I don't see Sega trying it if Nintendo didn't. Sega was always the more masculine 'cool-kid' choice, with Nintendo being the 'kiddies' console with wider appeal. The mascot being a girl might put off some of the existing market.
But, if they pulled it off, and Sonic lifts off the ground, then it may very well be Sega getting more widespread popularity. It probably wouldn't lead to Sega surviving and Nintendo falling off the world (thank the lord), but a larger, more diverse corps of games designers being raised on what I would admit to be a fine series couldn't be a bad thing.