There's still a number of To-Do liveries Hornby has yet to produce on the new tooling, and now they've finally tooled up the MML Variant then that adds more possibilities! Off the top of my head, they are;
GWT Merlin
FGW Barbie - and it's many, many variants (though one early build + one late build PC would be appropriate)
MML Ocean Blue
GNER - White Logo
We could also see another re-run of MML Teal and Tangerine with the proper lights, Virgin hasn't been released in donkeys years, and there's one off power cars like 007, 008, 102, 159, 184, 274, 277 etc. Another batch of XC Power Cars may be likely though given the small number now running around with Colas.
On the much missed liveries front - the 153 is another model severely lacking in new liveries too, though given Hornby's recent attempts on both the 153 and HSTs, people may be giving some of the more complex liveried models a bit of a wide berth.