The Healey Mills site is slowly being cleared but as it is a flood plain for the Calder, housing is unlikely. Returning it to nature would be more likely as nature has already claimed a fair chunk of it. A basic station with an island platform accessed by a ramp from the main Huddersfield to Wakefield road would be ideal to serve Horbury, Horbury Bridge, and Ossett. Couple that with the Horbury curve reopening and a large car park built, and Robert is your mother's brother...
Elland was planned to open with Brighouse but the funding ran out. Since then it's fallen down Metro's priority list for a number of reasons, not least a dispute over the site and a less-favourable business case than other openings such as Glasshoughton and the Aire Valley pair.
Seems a shame waste the land that could be used for redevelopment, has the yard ever flooded before?
As for Elland, I don't really rate Metro if I'm honest, they struggle to get the info right on the timetables at stops so getting stations and other major projects off the ground or they pick the wrong major scheme ie trolley buses. Every other big city gets trams and they want trolley buses in Leeds, ludicrous.
To be fair, Elland has everything going for it, just in the fact that every major town and in West Yorkshire plus Manchester and London would be accessible with no changes, that on its own makes the station an attractive proposition for the people of Elland. I know you can't have the 'build it and they will come' attitude but I do think you could have a little bit of that here just on the strength of trains going to where people want to go.
As for the Horbury curve, this could be tied in with the re-opening of the Spen line, another area with no rail head, Cleckheaton and Heckmondwike, in the Huddersfield and Wakefield direction would give Bradford access to South Yorkshire and the East Midlands without the need for a change at Leeds. I wouldn't trust Metro getting any of that to happen though.