Sheriff I think makes sense because Sakurai is clearly drawn to characters with a long history (Mr. Game and Watch, Pit) and he's had Sheriff as a trophy since Melee. The upgrade to Assist Trophy is a big deal, too, and shows that Sakurai continues to see the character as interesting and important.
Sakurai considered Excite Biker back when he was looking at NES characters for Melee, and his comment, something along the likes of "he wouldn't work because he would need ramps in the stage to jump! That's not actually the reason.
" Give me the impression that he more seriously considered Excite Biker and how he would work in Smash, and decided against it for some reason. Maybe just that he thought the tag team concept of ICs was more interesting.
Balloon Fighter was also considered in Melee when Sakurai looked at NES characters, and his reasoning was that he wouldn't be able to do anything without his balloons. Since then, we've had Olimar, who can barely do anything without Pikmin, and Rosalina, who is significantly worse without Luma. It seems like Sakurai has gotten more creative and more willing to consider a character with such a polarizing advantage/disadvantage state (Little Mac) since then and might be intrigued enough by the concept to take a crack at it again.
Takamaru is easily the most notable of the characters that are nearly Japan-only, and Sakurai seems to like the character, but has viewed the series as too obscure outside of Japan to include. Now that The Mysterious Murasame Castle has finally released in the West, maybe he's willing to reconsider. I don't think it's likely, though, and think the other three have better chances.
As for Mona, I actually just think she's more important to and representative of Warioware than Ashley is in the series. It's just that Ashley is more popular. If Ashley wasn't an extremely popular character and request, Mona would be a no-brainer. Since Ashley is so popular, it's a toss-up, imo.