I think the weird thing about Tellius banners is that I feel they kind of arbitrarily decide on whether PoR and RD count as separate things or not. I'm not sure if the fact that a banner is PoR or RD really matters a whole lot for New Heroes considering that before Jill and the GM deserters, the previous PoR themed banner was literally three years before with Elincia. But obviously we had a fair share of Tellius banners (even if beast units way oversaturated those banners for a bit) so it's not like Tellius fans were ignored in that time.
Like, they've been especially aggressive with Tellius units filling banner slots this year on seasonals with new characters so I think they're pretty adamant about this being a way to address that gigantic cast, and it's not like they're all just random throwaway characters either. Dheginsea had a fair share in the spotlight for the Halloween Paralogue, the TT ending dialogue for the Ninja banner delved right into Zihark's past, and in general there's been a lot of Tellius grail units this year too. Ashnard, Summer Rhys, Jorge, Petrine, and Halloween Ena, that's one Tellius grail unit every month for the previous 3 months now.
I think how they label the game mostly comes down to which game the banner theme better represents (Micaiah is obviously RD, Greil Mercs + Jill suits PoR), though the Gallia banner is highly debatable on that front. That said, they do seem to like throwing Tellius banners at us in waves, Micaiah banner wasn't that long after Elincia, and the two beast banners were extremely close, so I can definitely see them still throwing a more RD themed banner in the next few months. Tellius still has so many characters that can sell banners that they realistically can't be holding them forever, but I don't think it'll be necessarily because the last RD specific banner was in early 2019.
Ultimately I feel like Sacred Stones or Echoes is more likely than Radiant Dawn, but I don't think it's impossible for this banner to be RD themed. If I were IS though I'd probably throw in an Echoes banner here since they're obviously going to throw in a new OC on a New Heroes banner to prop up a game that otherwise might struggle to sell. And really, unless they change the banner philosophy this is the best time to add Echoes characters at all. Tatiana, Zeke, Luthier as a drop, and an OC is what I'm predicting for the start of Book 5, personally. There's almost literally nobody left from that game so if not now then I can't see another Echoes banner happening for a long time barring seasonals.