It would be interesting if they could do it in the far future....connecting Orangeville to Milton line would, by necessity also connect it to the Georgetown line and I like lines with multiple desitinations/options as opposed to single points.
The Milton Line is the busiest GO line after Lakeshore (West and East). They moved from 10-car consists to exclusively 12-car consists because of capacity issues and limited time slots (CP gave access to 6 now 7 AM/PM spots). GO goes where the demand is and it's not to Orangeville.
You have the minor works going on the line: PRESTO installations, Cooksville GO parking structure, etc. but beyond that, you need several large projects to run
anyextra trains. I would say all the following would be completed before running trains to Orangville on the Milton Line, but minimum of 1 & 2 would need to be completed first:
1) Don Valley River Bridge & Fly-under
2) 4 tracks to Meadowvale GO
3) 3 tracks to Milton GO
4) Milton PM Facility (Part of GO's breaking up of Willowbrook to service individual lines)
5) 4 tracks to Milton GO
6) 3 tracks to Cambridge
27% of people boarding Milton GO Station originated from Waterloo Region before GO began bus service to K-W. If GO were to offer Orangeville train service, the Georgetown line makes much more sense.
By servicing individual lines rather than requiring all train consists to cycle through Willowbrook Yard, GO will be able to offer more 'short turn' services that don't terminate at Union Station. This combined with bussing between train lines and local transit to offer an improving web of service. However, without the infrastucture improvements, you don't have any new trains going anywhere.