I've actually thought that a limited series featuring candidates from Series 1-6 would be a good idea, to give them a chance at earning the £250,000 investment from Lord Sugar. Here's who I'd cast from each series:
Series 1 - Miriam and Sebastian. Miriam's firing was one of the Sugar's all-time biggest cock-ups, which even he admitted, so she's a shoo-in. Sebastian was a pretty solid candidate all-around who could do with another chance to shine.
Series 2 - Alexa and Ansell. Ansell's a pretty obvious one, as he was clearly the most competent of that year's men and the one who genuinely wanted to work with Sugar the most. Alexa... well, I really can't think of any of the other women to bring back! Ruth's so successful now she could probably raise the £250k in ten minutes by calling her rich chums. Karen would never come back given how she was treated. Jo's too much of a loose cannon, and to an (admittedly much lesser) extent Nargis was as well. Sharon would be the obvious candidate, but I think her recent legal troubles would kill any chance of her getting the investment.
Series 3 - Jadine and Lohit. The former because Lord Sugar's said more than once what a great candidate she was and how she just came to the show a few years too early, so I'd like to see her put her money where Sugar's mouth is. Lohit, in addition to being a good candidate on the show, has actually set up his own online retailer, so he'd have the experience to make good use of the investment.
Series 4 - Alex and Sara. Sara actually admitted the job with Sugar would never have worked out because she prefers to be her own boss, so she'd be an ideal candidate for the new format. Alex did have some good moments on the show and hopefully will have improved with age - or at the very least, would provide some laughs by constantly saying "I'm only 30/31!"
Series 5 - Howard and Paula. Howard was a generally very strong candidate and also has experience running his own business, while Paula did a generally pretty good job as a project manager but got taken down by one absolutely horrific mistake.
Series 6 - Joanna and Raleigh. Joanna I think was unlucky to be in the last series before the prize changed; had the prize come in that year, and if she kept her attitude in check, I think she'd have walked it. Raleigh got taken out of the show by circumstances beyond his control, so I think he deserves to come back and have a proper go.