I couldn’t come up with what they’re talking about, but it ain’t good. Probably something about crushing dissenters and what to have for brunch.
 
I started this drawing and halfway through started questioning it. Nightmare Moon is still evil, power-hungry and arrogant and would have to fight constantly to secure her reign after deposing Celestia. She is a tyrant in every way. I have a hard time thinking Twilight would forgive NMM, much less join her. I’ve read a few fics like that, but it usually had a heavy dose of “Celestia was really a tyrant all along!” and I’ve never been a fan of that idea. The one exception was pretty damn dark and basically Twilight had a serious case of Stockholm Syndrome. So yeah, in the Starlight-altered timeline, Twilight would either just break and be a recluse, be imprisoned, or be leading the resistance. Maybe… Maybe if Twi tried to get close to NMM to sabotage her, free Celestia, etc…
 
However, I could totally see the above image happening if NMM had won that initial fight against Celestia 1000 years ago. I don’t think it would exactly be a loving relationship of equals though.