Yeah I agree, it'd be best to just do a fresh face... It worked for Mano, Fujimoto and Ayaya. They were seen on TV for the MM auditions, and only mano was an "egg" before debut.
Personally I think the solo idol wouldn't even have to be a triple threat in this market--someone like Uta who was subpar at singing and dancing probably would've made a good soloist just by her looks and aura alone. Out of all the recent eggs, I would have pegged Momohime the most eligible to go solo and I know for a moment after her Super Commander Momohime skit there were some people wanting her to debut solo... but alas. H!P saw that fan reaction of love for her and said "naaah let's just let her sit in KSS for another few years."
Let's face it, without the insight of Tsunku when it comes to member choices, H!P probably wouldn't risk debuting a solo idol. That's a lot of resources and risk to put all on a single person, and I have to imagine even the company realizes maybe they don't pick the best girls for certain roles anymore.