An AL1, an AL5 and an AM10, to current detail standards. An 86/0 type upgraded to Hornby's Class 50 standards would do though as it would be able to be released in a variety of Electric Blue and Rail blue versions.
Hornby have said they are going to do a Southern BR unit for 2006, they haven't decided yet what type. They were asking modellers for suggestions, who typically and despite Hornby stating it had to be a BR type suitable for the green era and capable of easy modification to allow other types to be modelled, trotted out all sorts of wierd and wonderful one off Southern Railway types, thus doing themselves no favours, and probably making Hornby wonder why they bothered to consult enthusiasts in the first place. My bet would be on an EPB or HAP type suburban unit, although there is some support for a CIG which just about fits Hornby's criteria. The CEP might just be in the running but would need massive changes to make it suitable for post blue/grey liveries, unless that is what Hornby meant by "capable of easy modification to other suitable types"!
Unfortunately it wasn't the best of models and it wasn't completely to scale, although at the time it was seen as good. I expect Bachmann will be announcing one after their year of consolidation, I don't know, I just feel it in my water. Something to do with having done the 2MT tank and 4MT Mucky Duck I expect!