I'd think a Class 08/09 could be a good shout - loads of variations and liveries, seen for decades all over the country. That sounds right up Accurascales street to me. As much as the Bachmann and Hornby models aren't bad, they're both getting pretty expensive for what they are and there's loads of oddball one-off depot pet liveries we've not seen produced RTR yet.
Classes 81-84 would be great, something not produced yet (or for a very long time in the case of the Class 81)
I'm also surprised that no first generation Cross-country DMUs seem to have been produced yet - Classes 119, 120, 123, 124 and 126 would seem a bit of a gap