If you really wanted to, and you had a good £500m to spend, you could extend the line to either or both of Kelso and Jedburgh. That's as far as it will ever go. There's no money in timber traffic. There are no towns that are worthy of the cost a brand new railway between Jedburgh and Carlisle.
It's a line that would have been nice if it hadn't have closed. But IT DID CLOSE. The only reason people like suggesting it is because it looks nice on a map and there used to be a railway line there. But rebuilding a line is about as expensive as building a new one. And it's not worth the close.
Any line that is ever suggested on the basis of "it would be a useful diversionary route" and "it should never have closed" is fundamentally not enough. It needs to stand on its own merits. This one doesn't, and unless a new town of 100,000 people is built in the Liddel Valley, it won't ever
Even if there was the money available, there are far more "worthy causes" than going all the way to Carlisle - unless mass timber traffic can be guaranteed for decades to come. The Buchan schemes come immediately to mind, and if we're in crayonista's dreamland there are sever shorter branches / extensions in the central belt that I'd put first. Let's start with Grangemouth, Penicuik, Kirkintilloch, Kilmacolm and Strathaven - the last if for no other reason than to hear people attempt to pronounce it (make it the terminus for trains from Milngavie for the jackpot!)
As to the others:
Penicuik isn't ever going to happen, the frequency could never compete with busses.
Grangemouth is unlikely, the connection faces the wrong waya and the station would be in an awkward place without some heavy work, although it definitely wouldn't be the most difficult.
Kilmalcolm and Bridge of Weir maybe, but unlikely. Neither of those places are really big enough for anything more than 2tph. It seems like a poor use of capacity into Central.
Kirkintilloch already has Lenzie station, and unless you build another station at Waterside, that's all it will ever have.
Strathaven isn't ever going to happen, the old bridge at Stonehouse is gone, and the one at Larkhall is falling to pieces, and any new line would need some seriously expensive viaducts.
Buchan I can see happening, but after the wires reach Aberdeen, as that's where the money is being spent for the foreseeable future.