Milford has been on the radar in the past, but certainly not Hopedale. And even at that, there are no current or active studies, designs, or funding for any extension to Milford at this time. I think there's more value in Woonsocket than Milford, personally.
At any rate, with the Fitchburg Line now extended and supported with a modern, expanded layover, there's three necessary layover replacements at this time. Extending the Haverhill Line to Plaistow and subsequent replacement of the Bradford Layover is no longer on the table. The Lowell Line extension into New Hampshire is also on life support, and only because of the City of Nashua, not the State of New Hampshire. So the Lowell Line likely won't be getting a layover at all just yet. That could mean the Franklin Line is next in line for a new layover, and the issue could be folded in with a feasible extension, as was done with the Fitchburg Line. There's already design work being done for Franklin Branch double tracking, which will be the next major double tracking project on the MBTA system. Perhaps there could be some momentum for further improvements.