Thoughts on what a deleveled mod would look like.
You are a mercenary, you work for cash. Everything flows from this primary motivation.
Act 1 will no longer restrict you to Watson. Feel free to go anywhere and do any gig. Earn money like the mercenary you are. Buy gear, get stronger. Come back and do the Heist when you want to make a bid for glory.
The player starts out as strong as they will end other than for gear (weapons/armor/cyberware). They will have what used to be perk-based abilities from the very start - air dash, deadeye or pyromania for example. They will start with same max health/stamina they will have all game, other than bumps from gear. Guns, armor, cyberware and mods will have no tiers or quality, a Burya will be the same as any other Burya in the game other than for any installed mods - iconic and x-MOD2 variants will still exist.
The economy will be the primary, maybe only, regulator of player strength. Purchase everything you want and need, kitting yourself like the merc you are: guns, weapon mods, armor, armor mods, cyberdecks, quickhacks, cyberware, consumables (boosters, drugs, food & drink).
The primary source of money will be quest rewards - you are a mercenary. You will not get richer through looting and selling stuff - you are not a trader or trash collector.
No more crafting, upgrading and dissassembling. Buy what you need. We could offer a form of vendor-based upgrades though - where you pay money to have your cyberdeck upgraded from a Mk 1 to Mk 2 - this would be cheaper than buying a Mk 2 directly. But there will be no upgrade materials involved - money is all you need.
No more perks. The player starts with all or most of the perk-based abilities active and available: deadeye, pyromania, adrenaline, overclock. (But some perk-based abilities may move to item mods or cyberware to facilitate choice and progression.)
There will still be a sense of player progression through gear (weapons, armor, cyberware, mods).
There will still be a concept of increasing difficulty based on the enemies themselves. Some will be tougher, requiring either better gear or more player skill to take down efficiently. Enemy strength will be based on the type - officers will always be stronger than mooks. We could go back to pre-2.0's level-based enemy distribution across the map, but instead of enemies having levels have tougher enemy variants where they used to have higher level enemies.
Enemies should be able to use mods and cyberware just like the player. Allowing them to use more abilities than just Sandevistan or Cloaking would be great - more parity with the player and more ways in which to control enemy difficulty.
Cyberware capacity will be max from the beginning. Likewise, all cyberdecks will start with the max 8 quickhack capacity. There will be more cyberware types available. Replace some perks with cyberware. Allow paying for surgery - add more cyberware slots in each body area. Let the economy (cost of cyberware) and slots (how many you install) regulate builds.
For consumables, replace the cooldown system with a toxicity system. Unlimited usage removes all challenge from the game. Cooldowns are very restrictive on the other hand. Toxicity-like systems walk a middle line that allows the player choice on whether to run the risk of overdosing if the time is right to do it.
For loot, have the player purchase what they need. Iconics/x-MOD2 may be distributed as the reward of specific quest types. But even here, letting the player purchase it gives them more agency - they get to maybe save up from the start for that sweet iconic they want to build around.
Remove all the incidental junk. They just clutter up the world.
Food & drink should exist in much smaller quantities, though this too is better purchased. Survival mods should plug right in, this mod should not itself handle survival as there are many different ways to do that.
For defense and damage, we may bring back armor instead of using perks to control mitigation in various trees - though it is also possible to provide perk-like, action-controlled mitigation through cyberware or mods. Maybe bring back armor mods as in pre-2.0. Maybe have all armor be granted through armor mods, with the higher armors costing more. All clothing will have the max number of slots, so you buy clothes based on looks alone. Let enemies have armor too. Find a good armor/damage formula (for e.g., Ghost Mode did this very well). (Defense/damage needs a lot more thought.)
The economy controls how you use all of the above. It's up to the player to buy what they think is most useful for their build. This gives build diversity through player choice of how they spend their money. This economy motivates doing quests/side quests/gigs/NCPD events to make money, not to get levels by killing more things.
There shouldn't be a motivation to kill everything you see - leave that to player choice, let them do it because they want to not because they need to grind up xp or loot weapons to sell.
If the reward is the same independent of how you do the quest, the player is free to play as they will.
The longer you play, the stronger you are because you earned and spent more money. You mercenary, you.
As you reach the end of the game you face stronger enemy types, but you can get past them through pure skill or because you spent a lot of time in the world and bought a lot of gear to make you stronger.
Levels would disappear from the UI. Under the hood, it may be best to set the level of everything to 60, the max value - since this may govern things like AI behavior.