Actually, one of my ideas involves 2D Shadow gameplay with guns. Here's how it would work.
-Summary: A hybrid of Contra style shooting controls (applies to all ranged weapons) with an element system like some of the recent Castlevania games.
-Details: Each weapon is of at least one attribute; slashing i.e swords, double glave (which is a long pole with a fang like metal edge, double implies one blade on each side), bludgeon (homing attack, club like weapons), piercing (spear, cross bow), ballistic (assualt rifle, SMG), fire (wrist mounted flame thrower), explosive (bazooka, etc), electric, and energy (Black Arms weapons). Each enemy and boss would have weaknesses and resistances to weapons of different attributes. To keep weapon groups balanced, each group also has its own strengths and weaknesses. Melee weapons would have good attack power and infinite ammo, but have short range and average cool down time between attacks. Ballistic weapons (relative to other ranged weapons) would have high ammo counts, the fastest cooldown, and excellent range, but the weakest attack power in the group. Fire weapons would have good attack power, and do lingering damage, but have short range and low ammo. Explosive weapons would have the highest attack power, excellent range, and good area of effect, but have slow cooldown times and very low ammo counts. Eletric weapons would have high ammo counts and the ability to arc over and damage nearby enemies with one shot, but their attack power and speed would be inversely related (need to charge them to get a good result). Energy weapons would have no weaknesses and few resistant enemies but no strengths and few enemies weak to them. Of course there would be unlockable joke weapons such as hedge clippers (slashing), spring loaded boxing gloves (bludgeon), frozen sword fish (piercing), rocket launchers that fire chainsaws that explode on impact (explosive, super ridiculous weapon), and the SBG 10,000 (parody of a weapon from Doom).