If I change one thing to make Battlemaster better, I would...

[trying]

Make the poisoned dagger actually kill people.

[SaDiablo]

I'm cheating and putting forth another idea.    Retirement for characters as an option rather than death.  To some that may help resolve there fear of character death.

[Vita`]

I'm rather sure you get a choice between retirement and death when you delete a noble.

[pcw27]

One or more continents should experience plagues.

[Vita`]

I would basically second Indirik's two suggestions. Increasing chance of wound resulting in death as a character ages and one noble per continent for every continent. I'd also say let everyone play on as many continents as they wish while we're in the same vicinity...

[Alpha]

I like Indirik's suggestion, though I wouldn't make younger characters immortal. Instead, give all characters a very low chance of mortality, from wounds increasing in severity, that increases every in game year, and increases more rapidly with age.

[Arrakis]

...introduce a deeper and more sophisticated battle system that relies less on who brings the bigger blob and relies more on tactical insights to win a battle. I am aware this is a coding nightmare, however, since the name is Battlemaster it sorts of makes sense to invest greatest efforts in that field.

[pcw27]

Customizable estates that have an impact on the region.

[Kai]

Sink anything to do with nobility and SMA. Back to TLs and communism and war islands.

[GoldPanda]

Get rid of the infiltrator class. 

[Daycryn]

I'd get more people to play it, and also a small army of developers willing to implement those features that are long in coming or so much work that they're not going to happen.

I realize that's two things.

[Sarwell]

More sophisticated infrastructure, and the elimination of the townsland/rural classifications (cities would remain separate), instead having something about infrastructural development that would allow rural areas to gradually be upgraded to townsland-level building capacities and townslands to be similarly downgraded.