The Interstellar Mining Consortium based on Bathos has detected energy signatures from a planet just outside of the current frontier.  Telescopic analysis suggests that this planet is being colonized by sapient aliens and that it houses the remains of ancient Carc technology.  Tsargis, our team has been hired by the IMC to recon this planet.  You are to assess the planet to determine its strategic and commercial value to the IMC and, if necessary, take actions to keep any Carc technology out of the hands of the aliens.  As you know, the majority of our resources are allocated toward fighting the Vatragyr, so you will have to scavenge for supplies on-site.    Do what you can to keep the aliens from acquiring and activating these artifacts.

You (the player, as the telepathic parasite and mercenary named Tsargis) start out with a loadout screen where you can pick a vehicle chassis and various weapons and accessories.  If the game includes the ability for the player to get out and walk around, this screen would be a good place to pick a host body for your tadpole-like form.  Your company's transport ship lands on the surface of the planet and you begin your mission.

My skimmer model has a pair of forward mounts for sensors or small guns; a pair of medium mounts that can hold booster engines, heavy missiles, mini-rocket pods, or large guns; and a pair of big thrusters that could potentially be swapped out for more advanced ones.  With the exception of thruster upgrades, all of these assets already exist, and many more can be made fairly easily.  In addition to missiles and direct-fire weapons like these, drones could also be deployed.

For how this loadout screen might look, SPAZ demonstrates: http://spacepiratesandzombies.com/mediawiki/index.php?title=Modules

Once you have your vehicle set up, you set off across the surface, encountering various battles as you go.  Instead of one-on-one, you can fight squads of human infantry and their vehicles in a more shoot-em-up style.  It might look something like this: https://www.youtube.com/watch?v=isZ_YtE6D70

As you defeat enemies and raid caches, you can potentially find salvageable weapons, systems, and ammo, or scrap to be used as currency for upgrades.  You can swap out the weapons and systems that you have, and anything you've picked up that you aren't using is stored on the transport.

How does this fit into the idea of a PDL?  The world need not be vast, but having it procedurally generated (or shuffled from pre-made parts) and procedurally populated with enemies (who can be modular like you) would make the playthrough different every time.  The game could be divided into levels, so there are various mobs throughout the level, with mini-bosses mixed in, and potentially a boss at the end.  Retreat is an option, but you'd better have enough scrap for repairs before you try again.  You could also go pick on some smaller mobs for a while.

The transport can't fly everywhere with reckless abandon, justified by it not wanting to get shot down, but it can land in certain suitable areas after you've cleared out the defenses.  Although there is permadeath, these landing sites serve as checkpoints in a sense because you can visit your transport for repairs (using what scrap you have scavenged) and refits.  The terrain of the planet is volcanic, with the various fights primarily happening in basins, craters, and calderas connected by canyons.  At some point through the "campaign", you could lose access to your transport.  This could mean going through an underground area, or having the transport captured or destroyed.  The transport could potentially also be used as a fast-travel system to revisit areas you've already cleared.

Player's vehicle: http://opengameart.org/content/modular-alien-skimmer-vehicle

Player's body (alternate versions):
http://opengameart.org/content/beetle-basic
http://opengameart.org/content/evil-cyborg
http://opengameart.org/content/rigged-textured-reptile

Player drones:
http://opengameart.org/content/spider-spy-drone-0
http://opengameart.org/content/sap-1-sapper-bot-mark-1
http://opengameart.org/content/ba-2-blast-all-bot-mark-2
http://opengameart.org/content/sentry-2-sentry-bot-mark-2
http://opengameart.org/content/spiderbot-10

Terrain assets:
http://opengameart.org/content/volcano-lava-floor
http://opengameart.org/content/molten-rock-like-thing
http://opengameart.org/content/3d-rock-tileset
http://opengameart.org/content/environmental-and-props
http://opengameart.org/content/low-poly-mushrooms

Enemies:
http://opengameart.org/content/abstract-rts-model-pack
http://opengameart.org/content/low-poly-fighter-pilot-rigged
http://opengameart.org/content/rts-modern-vehicles-tank-truck-harvester
http://opengameart.org/content/hover-racer
http://opengameart.org/content/hover-racer-2
http://opengameart.org/content/simple-skiff-optomised-and-textured
http://opengameart.org/content/future-fighterjet
http://opengameart.org/content/low-poly-modern-soldier-rigged
http://opengameart.org/content/tankcar
http://opengameart.org/content/t-3485

Mini-bosses:
http://opengameart.org/content/mech
http://opengameart.org/content/mcu-43-gryphon-mech
http://opengameart.org/content/floating-tank
http://opengameart.org/content/unfinished-mech-sketch
http://opengameart.org/content/mobile-turret

Bosses:
http://opengameart.org/content/sci-fi-light-tank
http://opengameart.org/content/thb-5j-thunderbird

Turrets:
http://opengameart.org/content/yet-one-more-turret
http://opengameart.org/content/turret-reviwed
http://opengameart.org/content/turret
http://opengameart.org/content/a-turret-again
http://opengameart.org/content/heavy-turret

Structures and props:
http://opengameart.org/content/turbine-textured
http://opengameart.org/content/sensorradar-tower
http://opengameart.org/content/reactor
http://opengameart.org/content/abstract-city
http://opengameart.org/content/sci-fi-crate
http://opengameart.org/content/game-crates
http://opengameart.org/content/sci-fi-containers
http://opengameart.org/content/pallet
http://opengameart.org/content/military-storage-box
http://opengameart.org/content/weapon-box-pack-3
http://opengameart.org/content/weapon-box-pack-2
http://opengameart.org/content/barrel-sci-fi
http://opengameart.org/content/3rd-person-blender-sci-fi-pack
http://opengameart.org/content/metal-barrels-pack

Music:
http://opengameart.org/content/attack-music-futuristic-pop
http://opengameart.org/content/planet-man-music-futuristic-hip-hop
http://opengameart.org/content/streetsound-music-futuristic
http://opengameart.org/content/poss-cypherfunk-music-futuristic
http://opengameart.org/content/echelon-music-futuristic
http://opengameart.org/content/bone-remains
http://opengameart.org/content/amiga-was-better
http://opengameart.org/users/clearside
http://opengameart.org/content/fight-them-until-we-cant

GUI:
http://opengameart.org/content/health-and-mana-bars
http://opengameart.org/content/some-hud-elements-icons-and-crosshairs
http://opengameart.org/content/simple-shooter-icons

Sounds:
http://opengameart.org/content/action-shooter-soundset-wwvi
http://opengameart.org/content/4-sci-fi-menu-sounds
http://opengameart.org/content/atmospheric-interaction-sound-pack
http://opengameart.org/content/space-ship-shield-sounds
http://opengameart.org/content/laser-rifle
http://opengameart.org/content/chaingun-pistol-rifle-shotgun-shots
http://opengameart.org/content/4-projectile-launches
http://opengameart.org/content/41-random-sound-effects
http://opengameart.org/content/robotic-mechanic-step-sounds
http://opengameart.org/content/the-free-firearm-sound-library

Misc:
http://opengameart.org/content/starship-corvette
http://opengameart.org/content/space-transport
http://opengameart.org/content/sci-fi-prop-set
http://opengameart.org/content/sci-fi-spaceship-ver-10
http://opengameart.org/content/harvester-spaceship-low-poly

Textures:
http://opengameart.org/content/camo-1024