The main problem I see with the two of them is JT needs to cycle the puck to be effective whereas Nylander is fundamentally suited to rushing and transitioning the puck up iceThey are just missing a glue guy.
They need a guy who can handle some of the dirty work but still has enough skill and skating ability to keep up. Kerfoot is not a great fit for the dirty work, but he is just overall a solid and smart player so he was still able to find some level of success on that line. Ritchie and others struggled to keep up.
A combo that actually worked was when Galchenyuk was on that line. Galchenyuk was not even all that great of a player overall, but he was able to be the guy which allowed Nylander and Tavares to both do their things. Drove the net hard and fought for pucks, which meant Tavares didn't need to do it all the time, but was able to be another option for Nylander on the rush as well. If you could find a better version of Galchenyuk to put on that line, you have a very strong line.
The line failed when Tavares was mostly just parked in front of the net and Nylander/Kerfoot struggled to get the puck to him or do anything else positive with it. The line worked when Tavares was more involved with the puck and was playing in space more often.
Malgin does have a nose for the net, and has a little bit more dynamic offensive skill than Kerfoot, so maybe it works. At the very least there will be a lot of puck movement.
They need to be separated imo