Lichdom: Battlemage doesn't have guns, but the spells are insanely customizable. The game is essentially Doom but every half hour or so you take a break from exploding undead, cultists, and demons to craft spells out of mook souls. Basically every review that gives this game a low score is because the reviewer doesn't want to learn how the spell customization works and doesn't enjoy taking at least 10 minutes out of every hour of play to fiddle with their spells. The developers added a "quick craft" button that'll attempt to create good spells of the selected types for you, but it's not very good and you're best off ignoring it and actually learning how the spellcrafting system works.
There's 8 different Elements a spell can be: Fire, Ice, Kinesis, Lightning, Necromancy, Corruption, Phase, and Delerium. There's 3 different kinds of spell: Targeted, AoE, and Nova. There's three different projectile types for Targeted spells: Missile, Lob, and Ray. Three different delivery mechanisms for AoE spells: Area, Pool, and Trap. There's only one kind of Nova, but the kind of Shield you equip (there's three) determines how your Nova is triggered, either on a perfect block, on a charged blink, or on both but you lose the non-charged blink. Each spell can be either Destruction, Control, or Mastery aligned, which changes how they function; Destruction spells primarily do damage, Control spells primarily prevent enemies from moving or attacking, and Mastery spells primarily apply Mastery Percentage which is consumed by charged spells to increase their effect magnitude by the consumed Mastery Percentage. Each spell alignment also has secondary effects based on the element of the spell; for example, a Kinesis Destruction Nova will knock enemies backwards in addition to doing damage, which is useful when using Shield types that trigger a Nova on a perfect block.
So: you choose three Elements to equip at once, and one spell of each kind (Targeted, AoE, Nova) from each Element for a total of nine available spells. For Targeted and AoE spells, you choose a projectile type, and for Novas, the kind of Shield you've equipped determines the delivery mechanism. For each spell you chose whether it does damage, controls enemies, or increases the magnitude of other spells you apply to that target. This is already a lot of customization, but I haven't even started on the procedurally generated loot.
You see, you craft each spell out of mook souls, and mook souls differ in both quality and kind. They can be Shapes or Augments; Shapes are Shields and delivery mechanisms like Missiles or Novas, Augments can be Destruction, Control, or Mastery aligned and go in the Shapes to determine the spell's effect and increase its magnitude. Shapes and Augments can have additional effects like increased critical damage, faster charging, increased status effect duration, increased movement speed on kill, increased projectile speed, etc. You get a lot of mook souls, so you're generally not hurting for spell components, and choosing which mook souls to put in each spell lets you tweak the magnitudes of each effect. To go back to the Kinesis Destruction Nova spell, putting in mook souls with high Destruction levels will increase the damage it does, but you might want to maximize the knockback effect instead by using mook souls with middling Destruction levels but high Control levels. If you want to maximize critical damage, you'll need to use mook souls with high Mastery levels, which generally means lowered Destruction levels, so you end up with a spell that doesn't do much damage on its own but has sky-high critical damage when you use it on an enemy with applied Mastery Percentage.
Lichdom: Battlemage is a great game, and if you like weapon customization I'd recommend you at least give it a pirate. It's got a new game plus mode, good graphics, and midriffs. Don't set it to the hardest difficulty mode on your first playthrough, though, because the hardest mode penalizes you for switching Elements and you'll want to give them all a try.