If this leads to Mother 3 and a new sequel down the line, awesome...
Don't hold your breath for Mother 4.
Shigesato Itoi has repeatedly stated that he won't make anymore Mother games. Yeah, Nintendo could make one without Itoi (probably with or without his blessing), but I doubt they would. The Mother series was Itoi's; I doubt Nintendo would make Mother 4 without him and anyone else trying to make it just wouldn't feel right.
Also, the games work really well as a loose trilogy. Ninten (and another party member) from Mother, while never explicitly stated, are speculated and hinted to be Ness' parents. The entire family has ties to Giygas. Then, a certain character shows up in Mother 3 as the main villain. If you played Super Smash Bros. Brawl or read this character's Earthbound wiki entry like I did, you know who it is.
Unlike Nintendo's other franchises, I'd rather there never be another Mother title, certainly not without Itoi and maybe even with him. I want to play Mother 3 desperately, but regardless of the ending, I'd be content leaving well-enough alone. It was written with an ending in mind. I look at the Metroid series and how Super Metroid could have ended the series. Metroid Prime could have easily been a reboot. As much as I like Fusion and don't hate Other M, they suffer because of Super Metroid's fairly conclusive ending. I wouldn't want to see that happen with the Mother series.