Some of my favorite games have involved a war mode that ties individual battles together into a story-less "campaign" and added an extra level of dynamics by way of bonuses.
The "Galactic Conquest" mode in the original Star Wars: Battlefront and "War of the Ring" mode in LOTR: Battle For Middle Earth 2 are good examples.

In SW:B (as in Evolve, now that I think about it), the player has a choice of which battle to fight next. Each location is tied to a map bonus, like improved health regeneration or extra ammo. If the player wins that battle, then the map bonus can be applied to next battle. After winning multiple battles for multiple bonuses, the player chooses which bonus to take into the next battle. That bonus is applied not only to the player-character but also to allied AI characters. The opposing AI army also acquires bonuses with map conquests. The maps are arranged in tug-of-war fashion, so an army can recapture a map it lost earlier in the war. The war is won when all maps are conquered.

BfME2's "War of the Ring" mode took it a step further with a RISK-style map. Rather than a simple tug-of-war, the war included multiple fronts. So focusing one's forces entirely on one part of the map could allow the opposing army an opening to flank elsewhere. Bonuses were cumulative, so the bonus of every conquered map was applied simultaneously (both +10 attack and +10 defense, for example, instead of either/or). That game involved more high-level strategy, but strategic weaknesses at the war level could be overcome by tactical strengths at the level of individual battles. In other words, you might enter a battle with the deck stacked against you, but by skill you win the battle anyway.

Whatever the form, a war mode enriches individual battles with a larger purpose and can add a lot of dynamics which keep gameplay fresh. The concept could be applied to both single-player and multiplayer (tournament) modes, but I much prefer playing against bots.