I've been thinking along the lines of this:
In terms of the masklings finding you, once the diplomacy update comes out there could be alliances formed between tribes, and some conversation could be exchanged about where you are. Also, I like the arena idea as a potential game mode for just practicing your strategic ability. super-small (10-hex wide) maps and placing in warbands would be a neat little feature. Also:
There should be some really advanced abandoned settlements laying about. You can capture them by simply building admin centres in the region, and it all is then yours. It can present a whole new side to "resource discovery based progression" if there are super advanced resources like Iron that can only be gotten from this one set of ruins, whose buildings are non-replaceable. Also, I feel like at the next opportunity to redo the maskling AI, it would be neat if you reworked it so that if they took control of one of your settlements, they would actually use it. Instead of just sitting there being confused, they could use your barracks to deploy archers, and use your towers to shoot your units as they attempt to reclaim your settlement.
Another potential side to this is; what if after capturing a different race's settlement and owning it for a while, you start to learn how to replicate their buildings? Say you added a new category, Experimental. If you've captured a maskling village and owned it for a while, you might get an alert saying "Your wise men have had an idea regarding Rat Riders and believe they may be able to breed them!". You will then have access to a high cost building in the experimental section, that may or may not work after you build it. Regardless, you can try again and again until you have a successful prototype, and from there can rule the map with maskling rat riders.
But the same could be true for the masklings. If they invaded your city for long enough, you could drive them out, only to discover they've begun to replicate your technology, upgrading their totems for more administration, building brickworks, and after that, walls, slowly growing to become the same kind of empire that would be capable of defeating you. I feel like this would be a neat change for the masklings. They would be far more sinister this way, going from the annoying tribal people to the annoying tribal people who will learn your technology and use it against you.
Sorry if this comment is kind of long, I just started typing and wrote a lot. Enjoying Boats and looking forward to Nobles!
P.S. (There's a small texture bug in 2.3.0 where if you place a wall at the edge of the map, it connects off to the infinite emptiness at the edge. It looks like this: