One extra bag space per X rebirths

Getting to RB5 is quick. Similarly with RB10. So, 1 bag space per 5 RB or 10 RB I think is reasonable. I feel 1 per 5 RB is a more gradual approach, and would benefit mostly players RB 15 and below. It makes the advantage less considerable for 20 RB, since they would have 4 extra bag slots while a 15 RB would have 3.

I also think it would encourage people to carry other consumable items like ginseng / scrolls / mana potions (for warriors) / or just extra health pots. Low RB pot chuggers would get typically 1 extra, which makes a negligible difference if they have 25 already. Carrying an extra 3-4 piece armor set is unlikely because weight is also a limiting factor; it would make it mandatory to have light on multiple armor pieces, thus specifically choosing light instead of endu / mcon / cic.

The only concern I see here is people carrying a bunch of invisibility potions. For this to work properly, DT potions should also exist, as was suggested in another post.