I was thinking the girls would handle their own vocals and play whatever instrument they choose on their respective album. They would collaborate with other musicians to create whatever sound they envision for themselves as an individual artist. I do imagine Rina playing not only drums, but also piano and maybe some guitar since she plays a little already. I also figure Mami would play some of her own drum tracks, and Haruna would play more lead on her own album. That's one of the details I think will be surprising for fans. We already know she's a good rhythm guitarist, but she does occasionally handle dual lead lines with Mami, like on "Your Song," and she quite possibly may have some skills tucked away itching to come out.
My idea of them doing a solo album, in keeping with the theme of KISS, is that the band is still together. This would just be a fun project for them in-between Scandal's official studio albums and tours. There are lot of successful bands that take breaks from each other once and while so members can explore music outside of the establish sound of their band. Def Leppard's Joe Elliot, Phil Collin and Vivian Campbell all do their own side projects between their tours and albums. David Grohl seems to work more often on outside projects than Foo Fighter albums these days. All four members of The Who did the same thing in the seventies, as did Charlie Watts and Bill Wyman of The Rolling Stones. Heck, Funkadelic was originally George Clinton's side project from Parliament. We ended up with two great bands in the end, and then one massive band.
In the case of KISS, two of the members did eventually exit the band a couple years after the solo albums, but that was primarily because they couldn't stay sober and were no longer reliable. I doubt we will ever have that concern with Scandal. Of course, judging from what we saw in the HELLO WORLD documentary, Rina has some high expectations, and Mami may need to be less laid back on workdays. Rina may be the youngest member, but I can see her as a no-BS band leader like James Brown or Frank Zappa. Her solo album will probably be completed first.
You are right, solo albums do tend to lead to a band breaking up, but this usually happens when that band has been together for a while, and the members are sick of each other and don't get along. Fame and success, or lack of fame and success greatly affects a lot of talented groups. Again, I don't see this being a concern with Scandal, at least not for a very, very long time. At the 10 year mark they seem to very much like and respect one another. How I wish I could have said the same about The Police, Sex Pistols, Guns 'N Roses and countless of other great bands that never hit 10 years.
Anyway, so that was what was in my head for posing the solo album idea. Quite a long explanation, huh?