So, I’m speaking as someone who hasn’t played ETG, but who has played a lot of rogues over the years. I think it’d help to have a sense of what you think you want to include so far in terms of mechanics. The core of these sorts of games is the question of how you become stronger, and the systems that you have to interact with to that end are part of what separates one from the next. Hades has its boons, Dead Cells has its weapons and upgrades, Risk of Rain has a bottomless bag of items for you to fill up…the list goes on and on. You may even have multiple systems that intersect and build on each other.
So, to start with, let’s just brainstorm a system that fits the weight gain direction for the game. Let’s just say you get an inventory with 4 slots. In addition to your weapons, you can also find snacks throughout the game, and each one gives passive bonuses, things like “more damage when behind an enemy” or “crits trigger AoE damage on the target.” But if you want to be prepared for the final boss, 4 snacks won’t cut it–to that end, when you find a snack you deem expendable, you can choose to eat it, filling up an XP bar that makes you fatter every time it maxes out and gives you more inventory space. The fatter you are, the more you need to eat to get more space, up to max level (or, if you want to go the RoR route, ad infinitum as you loop and get stronger until you choose to end the run.) You could instead choose to scrap items for currency, which you can spend at a shop for snacks or permanent stat bonuses, so you have to juggle these 2 resources–weight and wealth–to progress optimally.
That’s just one option–you could shirk inventory slots altogether and let players take infinite items at the expense of getting fatter with each one eaten, and getting fatter makes the game harder, so you have to choose when to scrap an item for money to spend on more power to accommodate more weight, and when to just eat an item for the bonuses.
More broadly, it’s important to think about the specific upgrades and weapons you get. One thing that sounds simple about a roguelike is the upgrades–just let the player choose between different stat boosts or simple things like applying debuffs, right? NO! I played a game recently, Blade Assault, that handled this very poorly. Items weren’t impactful, not only because they offered very small bonuses, but also because they didn’t really do very interesting things. They just offered little stat buffs for the most part, and this didn’t make them very compelling. Your upgrades should not only offer benefits in an interesting way, but they should go together in notable ways. In Vampire Survivors, for instance, the Bracer speeds up your projectiles, which doesn’t sound very useful until you remember that the King Bible can hit more times if it moves faster, and the Runetracer can cover more of the screen in the same way. So, instead of JUST having items that give you bigger magazines, maybe have others that build on that, like ones that deal damage to enemies as you reload based on your magazine size. Upgrades with a tradeoff are often even more fun because they force you to give something up–there could be an item that reduces you to 1 ammo, but boosts your damage based on your base mag size.
One last thought–it may be interesting to have different playable characters with different body types that you can take through a run. That can be your version of the different survivors you unlock in RoR, or the weapons you use in Hades. This can be a good way to let players nudge a run in a certain direction from the start–for example, you can have a really top-heavy character who gains a passive bonus to attack and reload speed at the cost of mobility, a bottom-heavy character with innate crit chance but also a chance to take more damage when hit, etc.
I’m going to push back against this because one interesting form of permanent progression could be making NPCs fatter as you get more loot. In Hades, you can renovate the underworld by spending the resources you loot during your escape, so this game’s version of that could be upgrading NPCs by simply feeding them.