Mini Campaign System

Some of these rules will look specific to BattleTech (because I'm trying to manipulate 'Mech stats into matching the fluff better), but the basic structure could probably be adapted to other game universes.

Before choosing forces you'll need to calculate three or four values for every 'Mech that will be available to the players:

Production Cost = [(cruising speed)*(armor points)+(jump MP)*(armor mass)/(mass of individual jet)+(heat dissipation)*(maximum possible heat)+(weapon and equipment BV)]/10
Ready Maintenance = 3*(number of weapons, equipment, jump jets and excess actuators) + 2*(maximum possible heat)
Active Maintenance = 1.33*(armor mass)^2/(engine mass)
Repair Cost = (C-Bill price)/100,000

These values are permanent traits of a given design and do not change during play. They're scaled so that a typical 2866-3050AD 'Mech battalion costs 3600 production points and is able to ready 90% of its force, deploy 67% an average distance, repair 22%, and replace 3% every turn.

At the beginning of every turn the players generate 3 objectives. Each objective has a random value and sits a random equidistance from both players. Roll 2d6 for each objective to determine distance, and roll 2d6*20 to determine their values.

There are two ways to play this:

Bean Counting Way: At the beginning of each turn, players add 3600 points (assuming that's how many they bought forces with initially) to their unused points from the previous turn. They must pay the "ready" cost for every unit they wish to use this turn. They must also pay an amount equal to the "distance" roll x "active" cost for every unit assigned to a scout force or a main force. Units which were inactive all turn can be repaired at the end of the turn. New units can be purchased for 8x their "Production" cost.

Fast Way: Before play begins, subtract the sum of each force's "active" costs from half the sum of its "production" cost; record the difference on a token. The force receives this many bonus points each turn (or, if negative, must pay that many points each turn). Use the 2d6*20 roll to determine the equidistance to each objective: if it comes up doubles, then players must pay 2x "active" cost to assign a unit to a scout force or main force; otherwise, pay 1x "active" cost. Deactivating an undeployed unit generates points equal to its "ready" cost. Units that were neither deactivated nor deployed get repaired for free at the end of turn. "Objective" points must be spent to purchase new units (at 1x "production" cost) the same turn they are won.

Now, roll 1d6 to determine the type of objective:

1-2 Stand Up Fight (whoever destroys the most units (powered down units count half) in 10 turns)
3-4 Territory (whoever has the most hexes closer to his ground units than to opposing ground units at the end of turn 10)
5-6 Grab (divide points into 4 equal lots on the middle of the board: any 'mech can carry points (on its back) equal to its tonnage, but if it doesn't have hands another 'mech would have to put them there; weapons fire destroys 1 point per point of damage) 

Once the objectives have been generated, both players secretly assign their units to any of three roles (scout, main body, rear guard) at any of the three objectives. The objectives are then resolved in any order the players choose. Resolve one site entirely before moving on to the next.

Any unit that exits off a neutral map edge is considered to have powered down or fled (as appropriate). 

Scouts

Determine maps randomly, then lay them single file, narrow end to narrow end. Both players reveal their scout detachment. The force with higher average speed gets to choose its home edge. Disregard any units still on the board after 10 rounds.

If scouts advance off the opposing edge, sum their Cruising MP (plus the range of any Active Probes, plus 1 for every .25 tons of "sensors"). The opposing player then matches that total by revealing Main Body units and summing their Cruising MP (plus the range of any ECMs on the revealed units). The "EW Suite" on a revealed RVN-1X counts as 4 points of Scout probe (even when deployed with the Main Body) and 6 points of Main Body ECM (even when deployed as a Scout). If the opposing player runs out of Main Body units, begin revealing Rear Guard. If the opposing player has no Main Body or Rear Guard, then the Scouts win the Objective.

For each 2.5 tons of communication gear that advances off the opposing edge, the player may ask one yes/no question about the opponent's Main Body or Rear Guard.

Any scouts that retreat off a friendly edge join the Main Body action, but do not count as "revealed" units or contribute to the Main Body's average speed.

Main Body

After scouts finish revealing units, both players secretly decide which of their unrevealed units they want to commit to the Main Body, which they want to send after the enemy's Rear Guard and which they want to send back to their own Rear Guard (units assigned to Rear Guard do not have to pay the "active" maintenance cost). Both players then reveal all Main Body units and any Rear Guard units (like artillery or Mobile HQs) that affect the Main Body engagement.

For each 2.5 tons of communication gear a player has in the Scout force or Rear Guard, the player may choose a single mapsheet. Determine the other mapsheets randomly. The Main Body with higher average speed gets to choose its home edge. Resolve the Objective as described earlier.

If a player sent Main Body units after the opponent's Rear Guard, then lay out a second map for that battle. The Rear Guard player chooses all mapsheets, reveals all forces, and deploys on the map. The attacking player chooses home edge. Play this battle simultaneous with the Main Body battle but do not end until one side or the other has surrendered or withdrawn from the Rear Guard maps.

If a player wins the Objective or advances units off the opposing map edge, then that player may choose to pursue any defeated or retreating opposing forces.

Rear Guard

Lay the maps single file, narrow end to narrow end. If the fleeing player already laid out a second map for the Main Body battle, then use those maps; the victorious survivors of that secondary battle chooses home edge and deploys their surviving forces opposite the pursuing player's home edge. If the defending Rear Guard has not yet been attacked, then that player chooses all maps and home edges and deploys the Rear Guard on the board (unrevealed units originally assigned to Rear Guard may deploy as Hidden Units). Otherwise, determine mapsheets randomly, and the force with higher average speed chooses home edges.

Fleeing units enter from the pursuing player's home edge. Pursuing units enter from the same edge on the following turn. The battle ends after ten turns (if not earlier).

Base Defense

To attack your opponent's resources directly, assign units to Scout, Main Body and Rear Guard like a normal objective. Then:

Bean Counting Way: travel the highest distance rolled for this turn x2 to destroy/steal points or inactive units worth 5x the total Cruising MP of your Main Body.
Fast Way: travel the highest distance rolled for this turn +2 to destroy/steal inactive units worth 5x the total Cruising MP of your Main Body.

Your opponent can assign "Base Defense" forces to Scout, Main Body and Rear Guard as well; however, the defender can reinforce the "Base Defense" Main Body by calling back Rear Guards assigned to other objectives. To do this, the Rear Guard unit cannot have been involved in a battle, and must pay the "distance * active maintenance" cost for whichever objective they were originally assigned to. (The Defender can also pay to "ready" otherwise unassigned units.)

VARIATIONS

Obviously my equations at the top there are aimed at 2866-3050 play, but higher tech shouldn't be too hard. Do maybe 6000 point battalions for the century before and after; count CASE as the equivalent mass of standard armor; make Gauss weapons add their damage to both heat and heat dissipation counts; I'm not sure how to account for gyro or cockpit variants. 

Could add forward bases to shorten travel distances and to avoid keeping your eggs (points) in one basket, or have your resource points travel with the units themselves. Another thing I considered was that any resource points that didn't get used by the end of a turn would move themselves 1d6 distance towards the opposing player, and would require their own set of "Base Defense."

It'd be easy enough to reflect the quality of the battalion's technicians, or their relationship with House quartermasters, by giving the player more or less resource points than were used for purchasing the initial force.

Raising the value of the Objectives, or lowering the price of replacement 'Mechs, would encourage more decisive battles (and fewer retreats).

[Edit, August 24: I've changed the "Ready Maintenance" equation so that LRM-5 boating no longer breaks even.]