I have a number of ideas that'll never get made. Mostly because the ideas are too weird, or too niche, to ever be considered by any developer.
In the 'too weird' category, there's, for instance, a vehicular battle royale.
Take Carmageddon 2 as the base premise, and make a single expansive map filled with roads, ramps, overpasses, all kinds of interesting features to drive around and smash cars together on. Gratuitously sprinkle pedestrians and squishy NPC vehicles all over it to act as easy credit sources. Add all the usual various powerups, with a strict limit of how many of what kind of powerup a vehicle can carry.
Most of Carmageddon's mechanics carry over with some alterations. Immediate-effect powerups that normally affect the whole map are instead active in an area centered on the powerup location. Upgrades are directly purchased with accumulated credits or found on the map, like in C2 proper. Autorepair is present, but cannot be used for some time after taking damage, or while driving. Recovery can't be used while near or in direct view of an enemy.
Buying and changing vehicles is possible (everyone starts with generic or mechanically similar vehicles otherwise), but requisite vehicles must first be found on the map hidden in nooks and crannies of the map, or gotten to as they're paradropped in. These new vehicles would be tougher, faster, more capable than the generic ones, but accumulating enough credits to purchase them can be an issue, and they would cost more to autorepair and recover.
Finishing off another driver awards credits and a kill as usual, but also all the credits and powerups that driver carried explode out of the wreck and are scattered around the nearby area as regular powerups.
The shrinking arena mechanic is handled a little differently than usual. The map is broken up into discrete chunks - sectors, districts, however you want to call it. Instead of a ring closing in on a single district, then, gradually the districts on the outer edges furthest away from the eventual last area, have the classic Carmageddon timers enabled. Anyone still in the district at that point, is literally running on borrowed time - they have only a short time to get out of the district or their car will shut down when the timer runs out. But, importantly, they can still extend that time by usual Carmageddon means - fighting, stunt driving, and running over pedestrians. This would allow short jaunts into the "dead areas" if need be. This feature will remain in effect in the last district as well, so that whoever stays there cannot just stay waiting to ambush their opponent with a slaughter mortar or something similar.
Overall, a fairly weird idea that isn't likely to ever be attempted.
In the "too niche" category, I still want to see someone make a proper, large-scale speculative sim game. Preferably combat-oriented, and would likely be at least in part an arcade sim at best, but a proper attempt at a speculative sim would be nice.
What I mean is. Take a game like, for instance, War Thunder. It's not really a sim in any proper way, but it's kind of aspiring to be - it's driven by verisimilitude, having things being a product of interacting systems and aiming to create a believable representation of those things as they could be or could have been, in a videogame. There's armor that tries to act as armor, there's some manner of simulation of projectile impacts, damage to subsystems resulting in believable consequences (even if dampened for gameplay purposes), there's all manner of physics systems and performance modeling, etc. I want to see something like that, but for things that don't, or maybe even can't, really exist. A military force composed of futurustic conventional vehicles like tanks and aircraft, mixed with combat mecha acting as IFVs and 'upscaled infantry', and VTOL strikecraft/"battlefield fighters" like in the "Echelon:Wind Warriors" game. Whether it'd be a multiplayer "team arena" like War Thunder, or a game in the dying genre of campaign-driven vehicular action (like the Enemy Engaged series, or the aforementioned Echelon), I'd really like to see someone tackle the issue of futuristic improbable/impossible vehicles never really getting the attention to detail in games, that they often get in movies or written fiction.