The Barossa line will not survive as a tourist line only.
I hope that there could be a farewell trip on the line . But yet again what is on broad gauge . Adelaide metro train or maybe a NRM Red hen special .
The Barossa line would have to be rebuilt to have even a tourist service or a 'farewell' enthusiast special, if it was in a fit state to take paying passengers there would be Tour Down Under specials operating every January. GWA doesn't believe in investing in their [leased] assets to maintain or generate business, they like to sweat the assets until they have no value left and then abandon them.
Is there any potential for suburban services in that direction?
There is very real potential for extension of the DEMU service to new stations at Concordia and possibly Kalbeeba, 3.2km and 5.1km along the line from Gawler Central at the edge of the Gawler urban sprawl.
This could easily be the best bang-for-buck rail project in the state. The lease on that section of the corridor could be bought out for $1 (overvalued for administrative purposes, the actual value to GWA is $0 at best if they aren't running trains on it), cheap and cheerful flat-pack stations erected (like single platform versions of St Clair), the track repaired and a couple of signals relocated.
The cost of the track renewal would be defrayed by using it as an exercise for training up a new in-house infrastructure maintenance crew. Once completed, they would then be able to be put to work doing regular maintenance (I expect to be shouted down on this count, unrealistic revolutionary proposals aren't really tolerated on this site) on the rest of the Adelaide Metro network instead of leaving the tracks to rot for 15-20 years until they need major capital works to rebuild from scratch.
As for extending Adelaide Metro trains beyond the Gawler urban sprawl into the Barossa Valley proper? No.
Or, to put it more plainly, is any attempt being made to beef up the population base in the Valley as has been done at Mt Barker? Or, is the real estate too valuable for that?
There is a growing resistance to development in the Southern Vales region which appears to have Planning Department support. The wine industry has a big pull.
All this suggests that any significant increases in town sizes in the Barossa Valley, east of Lyndoch', will face significant opposition. To expand the population to a size which justified a rail based public transport system would mean the valley would be taken over by tar and cement.
In both the Barossa (taking the loose definition of "anything east-north-east of Gawler" and not the actual definition) and the Willunga Basin there is also a fair amount of mundane agriculture as well as the wine industry. This is important for food security and should be protected until all of Adelaide has been through at least one cycle of urban renewal.
As I wrote already, extending suburban rail through the areas already lost to urban sprawl (i.e. to Concordia and maybe Kalbeeba) is very realistic, but it shouldn't go further into the Barossa Valley proper unless the locals show there's demand by packing the existing buses to the gunwales. Even if the demand materialised, it's doubtful that even a fully rebuilt rail corridor would actually present an improvement thanks to the twisty alignment.