I used a different approach to variable objectives, rather successfully though I say it myself, in the BBB Gettysburg and Koniggratz scenarios. In both these battles, the defending generals’ plans were messed up by their subordinates taking the initiative, advancing to hold what they regarded as vital ground, and disrupting the defensive deployment scheme.
For Gettysburg, rather than just have a “stupid Sickles” rule, I put in three possible objectives (in addition to some fixed ones like Cemetery Hill):
“Meade seriously considered trying to hold Gettysburg and/or to attack Lee’s left. He was dissuaded from these ideas, but Sickles unilaterally chose to advance to occupy the Peach Orchard. After both sides have deployed their reinforcements for Day 2, roll D6 to see which plan of debatable wisdom is adopted:
1-2: Sickles! The Peach Orchard is an Objective.
3-4: Hold Gettysburg! Gettysburg is an Objective.
5-6: Attack the Rebel left! Benner’s Hill is an Objective.”
Similarly for Koniggratz:
“Benedek’s dispositions were badly compromised by two of his generals, Festetics and Thun, who decided to advance from their designated prepared positions to the high ground overlooking them, thus exposing the Austrian right to attack. […] To reflect how the unhelpful initiative of the Austrian corps commanders dislocated Benedek’s defensive line, determine the objectives for Victory after the Austrians deploy, but before the Prussians do. There are 4 pairs of possible Objectives:
A The Holawald or Ober Dohalitz
B The Popowitz woods or Problus
C Maslowěd or Nedělišt
D Břiza or Stěžer
Roll D6 four times, once per pair. On 1-3, the first named location is the Objective; on 4-6, the second one is. Chlum and Sadowa are always Objectives.”
My approach was to make these objectives known to both sides, but you could equally keep them secret until game end.
I hope this helps. Good luck with your scenario-devising!
Bloody Big BATTLES!