Okay, so previous unnamed warlords, in that case....hmmmm....
I think so, if they're appropriate to a flashback. iirc (I had to research this about a year ago) we don't have an exact pinpoint on how long the warlord system has been in place, only that it started post-Roger. Therefore, if there's an event post-Roger that's seminal to the story and:
- Requires a flashback.
- Requires powerful characters to make the scenario believable.
Then I think Oda wouldn't have a problem drawing them, similar to how he depicted CP-9 in the Ohara FB.
Potential events would be:
- Something involving revolutionaries, specifically Dragon.
- The obtaining and/or transport of the Nika fruit.
Since their entire narrative purpose would be to showcase the strength of an opposing force (they're destined to lose since our futuresight tells us they're no longer involved) I don't think things would bode well for them.
Of course Oda could make one that wanted to retire, had to go under WG protection, etc. etc. but their most logical purpose would be to display that the WG wanted to accomplish something via strength, but an opposing force was able to take it down.
In that sense, and with the story ripe for flashbacks, yes, I think it's entirely possible and perhaps even 'likely'.