So I've been thinking of doing a homebrew for a while and I've got enough of an idea for how it would function to ask for some opinions before I start doing the more in depth stuff. Keep in mind that I've got nothing in terms of actual numbers for any systems or anything, I just have the idea of how they'll work.
The idea is a pacific rim style mech vs giant monster game. I know some of these exist already but none of them quite do what I want them to. The basic mechanics here are pretty simple, sort of an Xcom deal where the players start with a small base, low level tech and primitive mechs, and then via winning fights against stronger and stronger monsters, they level that base up, develop new tech and get better machines.
The Mechs themselves are based around a "Frame" a sort of base skeleton that is then customized. I've played with several ideas of how to make these things, from whole body frames to individual part frames (Ie you can switch off the leg or arm part of a frame and keep the torso, or something similar), but regardless of that decision, the frames determine the base stats of the mech, as well as things like available load and number of part slots.
From there players "slot in" the parts they want on the frame. So a basic frame might have 3 chest slots, 3 arm slots per arm, 2 leg slots per leg and 1 head slot, all of which act as places to put parts. Some parts, like the reactor and the cockpit are critical and have to be there, and generally in certain places, while others are completely optional. A cockpit has to be in the head or chest slots, a reactor has to be in the chest, but a flamethrower could go just about anywhere. The frame and the part both have HP which is added together to create the HP of that section of the mech. Armor can then be added on top of this, which effectively adds more HP and protects the function of parts from damage, so that sections can be damaged without losing functionality.
Once functional parts start taking damage, they have a chance to malfunction or fail when you try to use them. For instance, if I have the previously mentioned flamethrower embedded in an arm slot and it's been badly damaged, I'll roll each time the player uses it. If the roll comes up bad I'll roll again on a malfunction chart and see what happens. These effects could be many things, from a simple jam to a catastrophic and explosive failure. It should be noted that these "Malfunction rolls" apply not only to use of the functional part but also to the part of the machine its in. So if that part was in the left arm, any use of the left arm would have a chance of malfunction, albeit with a different chart of failures. For instance, it could have a power connection failure and the arm could go dead because the power was severed. These failures can sometimes be fixed "On the fly" if the players have the right cockpit parts installed. Like frames, cockpits have slots that can be filled with optional addons. For instance, a power control panel would allow them to take a turn to try and reroute power around the failure and fix the arm. Where as a Blast Bolt panel would allow them to sever the dead arm and gain enhanced movement because of the lighter weight.
Weight and power work much the same, though inverted. For power, you have your reactor outputting a specific amount of power while each part has a Min, Optimal and Max power it needs. For instance, I might have a laser rifle that requires 5 power at optimal, 3 min and 8 max. Below 3 it won't function, and at 3 it will have a -2 penalty to damage. At 5 it will do normal damage while at 8 it will do +3 damage over normal. If they have the right parts they can overload it, putting more than 8 in and getting higher bonuses at the cost of damaging the laser. For weight it works the same, but opposite; Frames have a optimal and max weight, with higher weights slowing them and lower weights making them faster.
Part of piloting the mech is controlling where the power is currently routed to, sort of like FTL. So you can shift power around to increase damage or speed or just get it away from parts you're not using. There's also the option to shift it to sections in order to increase specific stats, such as to the arms to increase strength and to the legs to increase speed.
The giant monsters will be handled in much the same way; with a "Skeleton" base and then organic addons such as horns, fire breath and the like, with different skeletons and parts locked off in "Tiers". I intend to also make a random generation system for the monsters that can be used to generate random monsters to fight rather than having to custom build every one of them.
Combat will be grid based and fairly standard in terms of the basic mechanics of how to hit and dodge and such. Simple opposed rolls with the difference between rolls acting as a bonus to set weapon damage. The more tactical aspect of the combat comes from targeting parts and limbs in order to destroy or incapacitate them. Each slot as a location on the frame (see shitty paint image), so destroying the shoulder slot of a mech will sever the whole arm, and likewise with a creature. Mechs have the benefit that they can only truly be stopped by destroying the cockpit or reactor while creatures can bleed out and die from limb loss and the like.