well i voted for classic space, blacktron, M-tron.
First, i feel you should pick a "heavy weight", not just a theme that lasted for one year and didn't make any impression at the time.
Second, we've seen several sets of pirates in the last 20 years. I really don't think Lego should use this format to produce yet another Pirate set.
Then "Castle"... you know, if Lego were to release a "UCS castle set" of 300 euro with at least two classic factions, sure, i'd be happy and buy it... however, you can use the "ideas" format to do the same. In fact, they just released a "blacksmith"...
so... ...
i think it's time to get Space in the picture. Space never sold really well, it was never the most popular theme, in fact, that still seems to be the case, even though the classic space dude has become a cult minifig...
But, i think many older fans are dreaming of a Galaxy Explorer 'reboot'... just for once, as a salute to the old theme. We also have not seen any reboot of blacktron 1 and M-tron for decades... A shame as well... so i voted for those three.
However, I'm guessing hoping it's gonna be Classic space, Adventures and Castle (whatever theme).
sne