1- Soft cap in range.
2- More complex missions that only do one thing all the time. Defend this, kill this. More Mechanics.
3-More resistant enemies. Still the perfect example is the nox i think.
4-Scalable rewards. That play against hard enemies will be rewarding.
5-Less stack % damage and maybe more additive, and less cheap, that only require put . One example you have difference of damage between 2 weapons, with mods this increase, with school focus this increase more, with arcanes this increase more, with buffs of warframe increase more, and optional but add rivens.