Well Stellaris isn't exactly your normal 4X. Those tend to be turn-based and Stellaris is real-time. Obviously, the exploration and (peaceful) expansion of 4X games don't fit into a BattleTech universe game, but the rest do. And Stellaris has to be mentioned because it's space mechanics would be the closely comparable to BattleTech. Gameplay wise, CK2 would probably be the closest. Combat would either have to be abstracted (like the earth-based grand strategy games), or an entirely new system that Paradox has never done before would need to be created.
Agreed that it would be great, but Paradox has explicitly said that they're not really interested in doing small scale spinoff games that don't have broad appeal outside the setting. This was in response to questions about whether they would do a March of the Eagles or Sengoku sequel/style game again. Both had some really interesting gameplay elements but weren't really popular. Paradox games are becoming more mainstream, but a Succession Wars game while sounding absolutely amazing doesn't sound like a game Paradox would do. Like I said, if HBS wanted to do it, I could see it. But I think it's very unlikely that Paradox does it.
