I've looked at this long and hard....and outside of a Watsessing extension I just can't see it happening. I'll reserve other judgement for the study whenever it comes out, but just opinion based on what exists now and my own speculation:
1) Watsessing Ave Extension theory: This should happen for multiple reasons. With Hartz Mountain gone, there has been some development on the line with the branch basically dormant, so the line is available. Between the Parkway Loft Apartments on the opposite side of the station and the open land with the demolished factory and such at the eastern side, there is a lot of room for transit-oriented development. While most of the TOD has been at Bloomfield station (which needs a renovation badly), there is open land for some at Watsessing. I for one, personally used Grove Street many times when I was at MSU, I didn't like its location, between parking and everything. Watsessing has a chance to be a more rational terminal for parking, development and growth. (Hopefully it would also mean a new bridge over the tracks with opportunity on the other side.)
2) Orange Branch between Silver Lake and Forest Hill. This is where it gets cloudy. In order to make a version where a train from downtown Newark gets to the Boonton Line stations, there is the issue of the junction north of BBP station. Right now, the only way you could get on to the old Orange Branch tracks is to deadhead to Silver Lake or Grove Street, switch positions and move onto the track at the switch facing the other directions. People more educated in light rail transit can probably answer this for me, but I don't think there is a single way this makes any sense in any fashion? You'd probably have to build a flyover ramp to connect from BBP to the Orange Branch, and clearance there is minimal. Now if they were going to consider Grove Street service via the Boonton Line only and make a forced transfer, this becomes easier and cheaper.
3) Forest Hill. A station at Manchester Place is likely, but I personally would prefer it at the crossing closer to Park Elementary. The track is in good shape here, and there is room for a small platform. The station at Manchester could be a big one for the Cherry Blossom Festival as it would be a block from the Visitor Center. A station at the old location of Forest Hill would be nice, but I don't see it working in any reasonable design, especially if you put one further down Manchester Place.
4) Boonton Line east of Forest Hill. 300ft east of the old switch at OJ, the track and ROW has met asphalt for a local company and been fenced off. Hints of the track are still there, but it's been encroached. The serious encroachment on the ROW goes for about 2000ft eastward to the site of the North Newark station. In the last year, they have torn up and fenced off most of the old North Newark platform, which used to be abandoned and publicly accessible. I personally don't know what their plans are for it, but obviously, it would need serious work. East of that, what kind of shape is WR in? Light rail vehicles are much lighter than commuter rail, but that bridge is 121 years old. East of that, the last time I was in West Arlington, the ROW was a forest. While the ROW is intact, is there really a demand for service at West Arlington and Arlington for light rail?
5) Boonton Line west of Forest Hill. As Hawaitiki's post above notes, there are proposals of a railroad track/rail-trail hybrid combo for the line. That would be a great idea. However, the problem we get when we return to Forest Hill is trying to get from the Orange Branch to the western direction. Similar to north of BBP, you'd have to build new tracks near OJ to facilitate service because again you'd have to make a turn otherwise or build a loop somewhere. In this situation, I can see a station at the old Forest Hill stop for the Tiffany residents, but again, I prefer the spot further to the southwest. There aren't any real concerns about the stretch until we hit Benson Street, where I'd be concerned of NIMBYism. Are Glen Ridge and most importantly, Montclair, going to support such a project? Having lived in Montclair/Little Falls as an MSU student, I cannot see that happening.
6) Terminal in Montclair? This one I think is the real killer. Aside of the NIMBYism, there is not a clear cut rail-oriented terminal in Montclair for such a service. The location of the Boonton switches is south of Walnut Street and I don't think Pine and Claremont is a reasonable terminal. Theoretically you could end the service at the Bay Street overpass, build ADA stairs to climb to Bay Street and manufacture the trip over. It's a 660ft walk from the station to the bridge, and you have extra parking on Highland if the speech/hearing center doesn't mind forking some over. I just don't see engineers/planners ever looking at such an idea though.
I personally would love service on both Erie lines back. However, I just don't see it happening in any logical way that doesn't cost an awful lot.
I didn't get into teaching for the promotions or the pension plans or so I could get to the golf course by 3:45. I did it because I wanted to help you kids. I'd forgotten that, till today. -- Principal Peter Prickly.