First, a quick summary of what facts I’ve gleaned from canon re Ratar-O and the rats, and a few assumptions I’m basing my headcanons on:

Ratar-O was the absolute ruler of the rats, but I'm unclear what his title is. I’m also unsure if he inherited his rule (as the heir to a ruling bloodline traced, at a minimum, back to Ratilla), or if he seized his rule on his own power. It’s probably splitting hairs, but I like knowing these things.

Perhaps worth observing: when Ratar-O states Ratilla is his ancestor, he could have meant it in a “the ancient rats are our ancestors” way, not necessarily a “Ratilla is my great-great-greatx50-grandfather” way. The latter has a stronger basis in canon because Ratar-O specifically states: “…My ancestor, Ratilla…” but, you know. Just putting that out there.

It’s unclear if Ratilla was already leading the rats or if he came to power after obtaining the Sword of Plun-Darr. I’m operating from the tentative assumption that if Ratilla was not “born a king,” then he established his dynasty with Mumm-Ra’s Sword. 

My tentative assumptions for Ratar-O are either: (a) Ratar-O is a direct descendant of Ratilla; and therefore Ratar-O inherited his rule, or (b) Ratar-O seized power and set himself up as the rats’ ruler by his own hand. I regret that I had to think so hard about that.

I’d like to see Ratar-O come back for a Round Two with Lion-O in either the second or later seasons, either as a lackey a la Slithe & co., or entirely independent of Mumm-Ra. I’d like to see Ratar-O moving against Lion-O on his own (they’re fun to watch together), but I think it’d be more likely for Ratar-O to approach the Ever-Living to ask for aid in revenging himself on Lion-O. I don’t recall that Ratar-O had a huge role in the classic series the way Slithe and Mumm-Ra’s other minions did, but I could see Ratar-O perhaps forming an alliance with Mumm-Ra rather than directly handing himself and his people over to Mumm-Ra’s absolute rule (as far as Rataro-O knows, anyway. lol)

Another thought I had is that Ratar-O might not return in the second season (although I do expect the rats to join the final battle against Mumm-Ra in the finale), but could appear as a villain in his own right in the third or later seasons, maybe in a kind of “I’ve been plotting my revenge on Lion-O all these years now feel my wrath!” way. That scenario could go all kinds of directions, and it’d make a nice early arc to keep the thundercats busy while Mumm-Ra is secretly recovering/freeing himself from the Ancient Spirits/&c.

Backing up a bit, I’m particularly interested in what Ratar-O’s people do in the wake of his defeat/the destruction of his mine: if the rats were abandoned by their ruler when he “fled back underground,” that may’ve created a power vacuum in the rats’ government, which would open the door to all the change and likely chaos that follows a deposed despot; or if Ratar-O only fled from Lion-O, but maintained his grip on his people, then there’s still the original question of the civil unrest resultant of tyranny, which could only be made worse by Ratar-O’s defeat at Lion-O’s hands, as well as general observations and talk about how Lion-O, an enemy king, treats the rats better than Ratar-O ever has (“Your people are just slaves on the other side of the whip!”)…

The rats clearly have a long, bitter history with the cats (along with just about every species on Third Earth lmao), so it’d be nice to see how Lion-O unites them under his flag (or his Code, I suppose), particularly since they have a very recent painful history in the cats’ enslavement and subsequent liberation. I wouldn't expect Lion-O to struggle overmuch to convince them to fight with him, but I would really, really like to see all the conflict that comes from the rats rubbing shoulders with their recently freed slaves, particularly through individuals' POVs. Such as, a cat (with their own story) recognizing a rat (with their own story) who beat him/her or killed a loved one or the like, and the many different ways similar scenarios play out across all these reunions. I'd expect to see healing if not forgiveness, eventually, but I wouldn't mind seeing any of the hundreds of other ways different dynamics end, either. And Lion-O getting caught up in at least one of these personal conflicts, and Lion-O talking to rats and cats personally and convincing then to move on, and just Lion-O being a fantastic leader to all these people with so much pain between them and hnnng I need it.

Now to conclude with a totally relevant gif.