They need to gate a lot more uses of skills behind training level, or, impose more risks for using a skill untrained.
A few random ideas off the cuff:
For example, if you try a combat maneuver using Athletic or Acrobatics untrained, you treat all failures as a critical failure. If you are trained, a normal failure leaves you flat-footed vs. the target. At Legendary, treat a critical failure as a normal failure (but without the flat-footed).
If you use a Recall Information action untrained, it takes two actions. If you're an Expert or better, it's a free action.
If you are a Master at Deception, you can choose to treat a normal success as a critical success, OR, you can feint as a free action at a -2 penalty.
I think the fact there were very few real benefits for Master or Legendary was due to signature skills limiting players advancing to those ranks for most of their skills. Now that anyone can reach M or L, it's more sensible to make achieving them cooler.
As it is, the change to untrained being level -4, while welcome, really makes it much more useful to use skill training to become trained in a new skill, instead of raising a skill to Expert or above.