Tony's comments on the solo album's style have been slightly contradictory - in one interview he said he'll stay in the rock/metal area, but in another one he said he'll explore some styles that wouldn't be possible within Sonata and he has lots of mellow stuff written. My guess is that it'll be something between those two: some rock/metal songs that don't fit SA and something completely different. Somewhere Close to You was originally meant to be on the solo record, but the other guys liked it so much that it was recorded for SGHN. In the Finnish magazine Soundi Tony mentioned that he'll probably only sing and play the keyboards, the rest of the instruments will be played by some hired musicians, because he isn't a great guitarist and can't play drums.
Initially I was surprised to hear about his plans for a solo album, as he writes basically all the music for Sonata, but I realized that he has such a diverse taste in music (he even likes reggae!) that maybe not all his ideas would be suitable for SA. It reminds me of the Steven Wilson situation: despite being the chief songwriter for Porcupine Tree, he has lots of ideas that the whole band wouldn't stand behind, so that's why he makes solo albums.