First of all I want them to sing musical theatre songs and second of all I want to focus on areas that we haven't seen much of yet, or that are potential weak spots.
Lee - we've seen Lee do practically everything, from heartbreak (he's very, very good at heartbreak) to psychotic anger (pretty good at that too)

. What we need to see is a big show-stopper piece, or something very light and comedic, something he hasn't shown us since Mac the Knife in week 1. I'd like to see him do a big number like Razzle-dazzle from Chicago, or perhaps Wilkommen from Cabaret. Or even the title song from Guys and Dolls. Or some Cole Porter (But I can't think of anything suitable just at the moment)
Ben - is turning into a superb performer, at least in the up-beat numbers he has been given in the last few weeks. We need to see, first of all if he can sing and secondly if he pull off something sad. If I really wanted to stretch Ben I'd give him 'if I loved you' from carousel. It is such a beautiful song, but one that needs to be sung so simply. Or, perhaps Maria from West side story, which takes a lot of skill. That said, I'm enjoying Ben at the moment and want him in for a little while longer, so I might compromise with Oh What a Circus from Evita, which is very different to the stuff he's been doing of late, but which I think he could do well.
Someone else mentioned Gethsemane, which would be a very interesting choice for him, I wonder if he could pull it off?
Keith - we know all about the big voice, so I don't think we need to showcase it. The worry with Keith is his acting. A big song and dance number, like the one's I mentioned for Lee would be interesting. I would challenge him with a dark, ironic song, something like 'if you could see her' from Cabaret. The other option is to go for something very grown up and suave, one of sky masterson's songs in Guys and Dolls, maybe My time of Day.
Craig - I like the idea of Mr Cellophane, but there are certainly more challenging options out there. What about one of the Judas songs from JCS? Or something from Phantom. Has he got that kind of range in terms of his acting? Craig's movement worries me as well, so perhaps something from footloose. I don't know why, but I've always wanted Craig to sing 'High flying Adored' from Evita, that's certainly back on safer territory.
Lewis - he's had plenty of earnest ballads, so we don't need to see that, and he's had upbeat poppy numbers too. Perhaps a big production number, 'I got rhythm' or something like that, or for something a little bit different, Sit down you're rocking the boat. I'd like to see someone sing leaning on a lamp-post from Me and My Girl, and Lewis would probably suit it the best.