Forte (a RoB/GBF/Shadowverse character) is very likely to come to Dragalia because she's featured in the Wyrmprint
Lord of the Skies, similar to how Cereberus (another RoB/GBF/Shadowverse character) was featured in
Fireside Gathering before becoming playable. Beyond that, many Rage of Bahamut characters were re-purposed as cards in Shadowverse as well as characters in GBF, so there's a rather large list of potential additions, such as Lucifer, Grea, Bahamut, Olivia, Anne, or even other Crystallians like Lily or Sky Knights like Albert.
If you weren't aware, the majority of Dragalia's designs for the cast itself are also re-purposed, from Cygame's defunct game Knights of Glory from 2013. So any designs from that game can also make their way over here as gacha characters or dragons. Some folks have compiled some comparison
images which are pretty neat.