I think we will not see it, at least not for timetable mode, because if you can change the consist then you change the physical properties of the train (such as weight, acceleration, max speed, train length) and the internal schedule and route prediction would become useless.
We would all like this feature, but, as Inkar implies, it would have to be part of an enhanced Scenario Planner, outside the current timetable mode.
Well it would be nice, just to sub in a BG vice a TSO on some of the Diesel Legends of the GW consists, so the trains are correctly formed. But as Crosstie says, the only way we will be likely to make our own consists is with SP2.0 which, as we know, has gone on the backburner while CC settles in.
