No new Dragon, true. I was thinking more as somebody else taking up the duties. Could be one of the other two (hoarding Sirio's Power) or could be a Non-Dragon successor, with the implication that all the Dragons' Duties have slowly migrated to other creatures. Since the Dragons used to be in charge of Season per WOJ, it could make sense that as they Die off other agents have ascended to their roles, somehow.
How do we reconcile Ferro caring about Sirio dying then? Separate WOJ attributes his reaction to basically 'Ferro would not have cared about some little D dragon nobody'. That being the case, if someone did step up to ascend to the role or duties then what does it matter to him if Sirio died? Coupled with the third WOJ that you essentially can't just go around saying you killed a Dragon because those in power will think you're insane...upsetting the 'status quo'...that if someone else took the role it ought not to matter.
Which is also why I don't like the idea of Dragon's being mantles. Too many ppl per in-book reaction and WOJ seem to be averse to what happened to Siriothrax dying that it doesn't make sense if the role just got interchanged. No need to be as bent out of shape about it then.