I don't know where I fall on all this yet, mostly sharing general thoughts and playing a bit of devil's advocate.
Cave-ins
Simplify to two rules: walls don't cave-in, and dirt can cave-in at any time (checked per turn and per move)
Mostly a player nerf because digging is extremely strong, but some flexibility is offered regarding walls
This would be a major nerf to current stealth builds and one that creates a risk that players should probably never take, given that these stealth builds often have several essential but weak parts (flight units, sensors, processors, etc) and little storage for backups or temporary removal, and losing one of these pieces can be detrimental to a run. This kind of risk already occurs when put in a position where you have to take shots from enemies, but is much worse in the case of a cave-in, which I believe does not respect part coverage. Combat builds likely to not care about this risk, so this continues down the path of nerfing flight builds (the primary users of stealth tactics and digging); but feels like a step too far, as it is likely to remove the "digging through dirt" option for many players (which is fine if this is the goal, but it seems like the current game design wants multi-tile digging to be an option).
On the other hand, this restores one of the larger impacts of the recent melee-digging nerf, which made it very difficult to destroy reinforced barriers in common prefabs where the reinforced barriers are surrounded by walls. Melee weapons are one of the best ways for destroying reinforced barriers (especially for faster bots with momentum bonuses) -- towards the end of the game, very few ranged weapons are capable. The melee-digging nerf has made it so that in many of these prefabs you now have to risk a cave-in as you are melee-attacking from one of the adjacent wall tiles. If walls never caved in then you could still attack from them.
Additionally, having walls never cave-in would restore the killhole tactics that were the source of the melee-digging nerf, albeit in much more limited locations. Another option would be to make both walls and dirt always be unstable, which would eliminate this.
We just got a digging nerf that many players are opposed to (I am not one of them, I think it is a fair nerf and probably makes these situations more interesting overall). We should wait and see how the most recent digging nerf plays out before further nerfing it, but these discussions are good to have.
Makes digging in caves much worse, probably a good thing as it trivialises caves
Digging in caves makes them much easier for experienced players who are taking advantage of sensors, optical arays, drones, or other data; allowing them to potentially dig around known threats. But this is actually not such a simple thing for newer players (many find the caves maps difficult based on what I see on discord) and rewards players in general for developing map sense (something that the caves reward in general, which very specific and predictable layouts for those who understand the maps). If we were only balancing for the top percentile of players then nerfing this aspect would be fine, but I think this is a good feature for the game overall.
Legs
10% accuracy malus and evasion bonus per point of momentum (caps at 30% or 40% with reaction control)
Accuracy malus uses movement acc malus, so it doesn't affect melee attacks
Only relevant when moving with non-overweight legs
Moving with legs on previous turn locks in malus for current turn
Bonus only takes effect during enemy actions when previous turn you moved with legs
I really like the general idea behind this and there are a number of ways it could be implemented. I like the relationship with momentum because it makes reaction control systems have more synergy with legs, which were already decent with legs but this improves that in a good way. Plus a lot of the leg ideas I've heard have seemed out of Cogmind's style, too similar to other game features ("siege mode, but legs!"), or somewhat ridiculous. This feels reasonable and fits the game nicely.
Overweight penalty
Increase somewhat for treads (maybe wheels too?)
Support should be relevant in every build, and treads are a bit strong right now
I would at least double the penalty from treads (20 -> 40), possibly more but 40 is probably a good starting point. Current overweight penalty for "basic" treads is 160 speed to 180 speed, which is only a 12.5% increase in time/move to gain *double* the mass support, which is significant on treads. Perhaps adv. treads should have a lesser penalty (30?) since being a little faster is their niche and this seems to hurt them more than other treads, although I think them being fast while not overweight looks pretty good already.
I'm not sure that wheels need a nerf in general, they can be quite fast if you limit your mass and evolve enough slots, but that's about all they have going for them (low coverage is a nice but is also a downside). Centrium wheels could possibly be nerfed as they are quite good and relatively common, but I'm not sure if it's necessary. If we are looking to nerf wheels then maybe tweaking the rating or rarity of centrium wheels could be a starting point?
Matter and energy storage utilities
Make it increase maximum capacity when equipped but not store anything when unequipped or dropped
'Drop on floor' strategy is extremely strong
I think there's some good merit to this idea, as sapping energy off the ground *is* quite strong, although it does come with some side-effects vs equipping (you need to have space to drop it, it only restores when the turn increments so you may have to wait to refill, etc.). There's also some awkward behavior if you have empty storage in your inventory, because you will sap matter/energy from storage on the ground into storage in your inventory, and not always gain any benefit to your useable active resources.
Another option may be to put a cap how much you can pull from the ground, maybe to something like 100/turn. I'm actually not sure that matter is quite as exploitable as energy, which could make balancing them both but maintaining some level of consistency difficult.
I do like the idea of being able to store energy/matter in your inventory still. Another option could be to halve the storage while in inventory, so they are more effective while equipped.
Does energy/matter storage share the property of inventory storage units where they can't be dropped due to corruption or severed by slashing damage? That would be necessary for this change, otherwise losing an equipped energy/matter storage part and watching as potentially 1000 energy or 500 matter vanishes could be devastating in a way that isn't very fun. This could be very nasty in
A0 where you are repeatedly ambushed by slashing attacks.
This also exacerbates an issue that many kinetic builds have in
A0, where matter can be difficult to come by (especially when the Architect constantly consumes resources without dropping matter). It's an issue I've run into a handful of times and essentially losing at this point in the game because you don't have enough matter isn't very fun. Perhaps I should just "get good"
If energy storage were to always consume a slot regardless, there is going to be a breakpoint here where evolving more power slots or using power amplifiers is better than having energy storage equipped. I haven't looked at numbers on this but depending on where this lands, energy storage could become somewhat irrelevant to a lot of builds that used to use it. This could also make endgame builds more interesting where you can't rely on large stockpiles of energy storage in your inventory. Any potential Storage nerfs will also affect this by putting more stress on inventory slots that could be dedicated to energy/matter storage.
Cave walls
All cave walls are earth
Fixes the advantage tiles has over ascii
This has always been an awkward problem since it is one of the few (if not the only) places where tiles have a distinct advantage over ASCII. Tiles revealing more information than ASCII is common in roguelikes, for obvious reasons (that isn't to say it should also be the case in Cogmind). Removing cave walls would make digging easier (especially from stray shots, which I assume is one of the reasons the walls exist to begin with). It also makes sense that walls would be harder and more compacted than looser dirt inside. I do wonder how caves might look if segments of the current wall tiles were made into dirt, so there was a mix while you explored, and it would also make the walls less reliable for guiding digging routes. If cave walls became earth it would push players to spend more time digging (essentially digging out every tile in a 3-tile radius to find an opening, instead of just 2), which would cost more time (not always a critical resource in the caves) but be a potentially tedious behavior.