Sorry to spam you a little bit with my wild ideas, but hey, WH40K is such a fun universe. Been reading some Lexicanum lately, very impressive well of lore. Anyway, I have been playing around with some numbers and I came up with an early version of the squad assembly material, mainly focusing on the pointsworth of each unit variant.
There are five unit types, as given by description:
(1.) Leader is an avant-garde Space Marine soldier in terms of battlefield prowess and biotechnological engineering. Distinguished with wearing an emblematic bone-white armour, the Leader leads battle brothers by example. Only one Leader per squad is allowed.
Special rules: twice per mission, the Leader may attack with ‘Frag Grenade’ (short range, single target, damage 4, blast, cost 4 AP).
(2.) Troopers are highly trained Space Marine soldiers, holding universal applicability of tactical value on the battlefield.
(3.) Medics engage in warfare to actively support their fellow Space Marines, helping many survive the wounds otherwise possibly fatal.
Special rules: each allied Space Marine on a tile adjacent to the one of Medic, receives a continuous bonus of 2 HP extra. The bonus lasts as long as the unit remains in touch with the Medic. Disjoining from the Medic, will make the bonus nullify as soon as the next turn (may cause death if little HP remaining).
(4.) Gunners provide a vital firepower backbone in the Space Marine army, excelling at heavy weaponry of devastating power.
(5.) Reserved for open field battles, the Raptors utilize their ‘Raptor’ engine jump-packs to rapidly confront the enemy and provide combat assistance of high mobility value.
Special rules: movement costs 1 AP per tile (unit rotation costs without change).
So these are the unit types. Now for the pointsworth:
Leader – Bolter Pistol: 4
Leader – Storm Bolter: 6
Leader – Plasma Pistol: 5
Trooper – Bolter Rifle: 9
Trooper – Graviton Gun: 10
Trooper – M/C Bolter Rifle: 15
Medic – Bolter Pistol: 13
Medic – Storm Bolter: 15
Medic – Plasma Pistol: 14
Gunner – Heavy Bolter: 11
Gunner – Assault Cannon: 11
Gunner – Plasma Cannon: 17
Raptor – Bolter Pistol: 14
Raptor – Storm Bolter: 16
Raptor – Plasma Pistol: 15
The squad at minimum consists of a Leader and two other units. Maximum is a Leader and eight other units.
Cheapest squad setup comes at 22 total pointsworth. Most expensive squad setup equals 142 pointsworth. I propose to introduce an order of thresholds varying by 15 each level, starting with 30 and ending up with 120. That would be: 30, 45, 60, 75, 90, 105, 120.
For each mission, the player would choose the threshold amount of points oneself. If mission successful, it would be booked in the high scores of given mission, allowing the player to sign in a name of the squad that made themselves a name. If the mission was completed without the loss of any squad member, the name of the squad would go to a separate - more important - book of high scores, marked as "perfect".
If you would like to know how did I get to the particular pointsworth values, I can link you up the spreadsheet. It is all rather nominal, we will see whether it does justice.
I think majority of typical missions should work on the basis of 'Capture the Flag', which means, not only to get something, but also bring it back to the dropzone. 'Assault' missions, are to achieve certain spot, but also to hold it for a number of turns. Another kind of mission is 'Headhunt', which ends with certain unit being killed.
Yet another type of mission is 'Mayhem', which ends with having all enemy units killed or rendered armless - the latter, due to shortage of both the ammo and the combative special abilities, for example - albeit this is no good against the endless spawnings of the Tyranid, so rather perhaps against some more quantitatively defined forces, such as other Space Marine Chapter. It is possible for the 'Mayhem' to end up in stalemate.
'Skirmish', is a type of mission which must resolve within fixed and narrow amount of turns. The party which has scored more pointsworth in slaughter of enemy units, wins. Again, this poorly applies against the Tyranid, but maybe if to assign each Tyranid infantry unit of any kind the symbolic cost of 1 point, the mode could be tested.
Summary of changes to the earlier assumptions:
=> Captain/Leader - changed the unit name to "Leader"
=> Leader - added special ability, the "Frag Grenade" attack
=> Medic - changed the rules of special ability
Furthermore, perhaps if to bring the base HP of the Leader up to 6, for the reasons of correspondence with the damage line of Heavy Bolter, such as the base Marine HP corresponds with the damage line of Bolter Rifle.