Definitely not happening since it was never beyond the planning and concept stage.
I don't know what you're talking about, Dave Filoni told plenty of original stories in TCW that added to the universe and weren't "backfill", many utilizing completely original characters like Ahsoka or Rex, or deepening existing characters who were barely more than set dressing like Plo Koon. Plus, even if you want to count Anakin's development in the series as backfill, it absolutely completely legitimized his character in a way the movies never did.
He was obviously hamstrung with Rebels in several significant ways, and he had almost nothing to do with Resistance past the idea for it.
At the end of the day, he was Lucas' protege, and the only one still at Lucasfilm who is allowed to have high creative involvement that seems interested in trying to take stories in this franchise in interesting directions.
I know. I'm not talking about an 8th season. I am talking about more episodes in the 7th. Ie, 4 - 8 more episodes (1 to 2 more arcs) on top of the 13 announced ones. It's unlikely, but perhaps possible. When they discontinued the series before, there was 9 different arcs partially completed (and more in planning). We were shown many of these half-finished arcs in the intervening years after it was cancelled and before it was announced as coming back. We know for a fact that 2 of the 3 arcs in the new season are two of those unfinished arcs (Bad Batch and Ahsoka in 1313), one of which they actually completely released in the state they got it to (Bad Batch) -- they also released another arc in the same state (Crystal Crisis on Utapau), but that one does not appear to be slated for remake.
The other unfinished arcs we have seen parts of include:
- Son of Dathomir (Which I would do anything to see animated -- it was released as a comic, but is incredibly integral to Darth Maul's story and would be a shame to have it trapped in that medium forever)
- Dark Disciple (Actually two arcs/8 episodes about Asaaj Ventress -- it was adapted into a novel)
- Cad Bane and Boba Fett Bounty Hunter Arc
They also talked about several arcs they never got around to even starting, but were planned for the final two seasons (the plan was always 8):
- Kashyyk Arc
- Rex and R2 "Top Gun" Arc
- Yuuzhan Vong Arc
- Sith Temple under Coruscant Arc (it is suspected that parts of the ideas for this were repurposed for Rebels episodes)
- Second Bad Batch Arc
- Return to Mon Cala Arc
- Siege of Mandalore Arc (confirmed to always be the finale, and confirmed happening for the new season)
If you count it all up, if each of these arcs were 4 episodes, that is 48 more episodes that were planned to the end.
Season 6 was cut short at 13 instead of the normal 20ish. So this is pretty much everything else that was planned for the series.