... hehehe. Well, in DnD there always seems to be a favoritism towards who gets the kill. Now, I am fine with experience being split amongst the group only based upon % damage ... however I want for number of kills to effect the total experience somewhere, so in that case I would want for total "battle experience" to equal Enemy Combat Value x 2 x (1.1)^N where N equals total number of kills.
By keeping the Kill value present "somewhere", even though sending Hoards of weaklings to their death is both fun and effective, it will increase the amount of experience your opponent receives.
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Now, onto your question of "how" leveling up should effect a unit. We already have a fairly good method for leveling up our Sovereign.
I will postulate the rest of my idea based upon the Sovereign method of leveling up.
Sovereigns get 10 points, Champions get 8 points, Elites get 6 points, Experts get 5 points, Veterans get 4 points, Regulars get 3 points, Draftees get 2 points, and Slaves get 1 point.
I would advise re-naming Veteran to "Honor Guard" and renaming Expert to "Exemplar"
I would love if each weapon has 3 skills (2 useful skills and 1 "super skill"). Regular units gain a skill each 5 turns, with the Super Skill being gained at level 15. Draftees and Slaves can never unlock skills. Honor Guard start with first skill, Exemplars start with first two skills, and Elites start with all three skills. Champions also start with all three skills.
Base HP for a default Champion should be 30, and base HP for a default Sovereign should be 40.
(base HP for regular is 10, base HP for Honor Guard is 12, base HP for Exemplar is 15 and base HP for Elite is 20)
base HP for Draftee should be 8 and base HP for slave should be 5.
Champions should have about 20 or so "feats" to choose from. 10 or so of these feats should be "generic" feats ... like stealth, hardy, Trollskin, ect. The other 10 should be dependent on Weapon-type ... meaning each "type" of weapon should have approx 10Champion feats attached to it (more or less depending). These would include Point-Blank shot, Rapid Shot, Long Shot, Precision, ect (Bows). Also including Dual-Wield, Parry Master, Nimble Blade, ect (Swords), Sweeping Strike, Knockdown, ect (Giant Weapons), Armor Smash, Excessive Force, Concussion Strike, Disarm, Trip, ect (Blunt Weapons), Disarm, Trip, Sweeping Strike, Arrow Block, ect (Staff Weapons), Excessive Force, Armor Slice, Decapitate, Dismember, ect (Axe weapons).
For specialty feats like Decapitate, Dismember, Disarm, Trip, Concussion Strike, Knockdown, ect ... would depend on relative levels, relative rank, and probably have some Champion/Sovereign only feats that allow full immunity towards said effects. (as well as many monsters being immune).
You probably would have 10% penalty for trying to specialty feat each rank above you, and a 5% penalty to try and specialty rank each level above you. With like-minded bonuses for specialty feat-ing ranks and levels below you. Trying to Specialty feat a Sovereign is an additional 20% penalty.
Ranks are Slave -> Draftee -> Regular -> Honor Guard -> Exemplar -> Elite -> Champion -> Sovereign.
So a level 5 champion would have a 10% penalty trying to specialty a level 1 Sovereign. While a level 1 Sovereign would have a 70% bonus trying to specialty feat a Regular soldier.
Lets say that Knockdown and Trip have a 40% default success rate (enemy loses 1 combat turn). Concussion Strike has a 30% default sucess rate (enemy loses 3 combat turns), Disarm could have a 20% success rate (enemy loses weapon ... 5 turns? whole battle?), Dismember could have a 15% success rate (enemy permanently loses an arm, effects stats accordingly), and Decapitate could have a 10% success rate. An unhealed Dismembered soldier could have 5 combat turns before it passes out/ dies. If Dismember successfully activates twice, both arms are removed, and if it activates a third time (upon the "somehow still alive" soldier that lost both arms) then it acts as decapitation.
Meaning, that if a soldier loses an arm, it has 5 turns to live. If it loses a second arm the next turn, then its total "dying" speeds up by 1 turn, so it has 3 turns to live (instead of 4). If dismember activates the next turn (on the same unit) then its head is cut off, and it automatically dies. A soldier with no arms, and no attack value anywhere else on the body (Bite, Feet-claws), it will either remain in rank/file, or flee.
If a soldier is healed by magic before the 5 turns run out (or its head is cut off) then it will resume normal combat ability. If a soldier has "died" due to the timer (and its head was not cut off) then it will lie passed out on the ground for another 10 combat turns. If there is any magic capable of healing it, and its cast before the 15 turns are up (or the 10 "on the ground" turns are up) then the soldier will not die, but will remain passed out for the rest of the battle (or 50 combat turns). To return a healed, yet passed out soldier to combat ability, you need to use a spell that wakes sleeping units.
A successful Decapitation immediately kills the unit, with no chance of magical healing (unless you have some sort of resurrect spell). If you DO have a resurrect spell, you can only use it once per battle. So you can either use it during the battle, or immediately after the battle is concluded (if you won). Leads to more interesting decisions imho.