As the title, I'd like to mod the current proficiency rules slightly, by the following two tweaks...
1. Adjust the game's expected DC table to assume a hypothetical (but unexistant) 3/4 level growth on most relevant rolls, instead of full level.
This will allow those who invested on proficiencies of any kind to feel actual improvement, as they have their required natural X value on their d20 dice gradually get smaller (about 5 higher, in this case). The old d20 system's massive bonuses, and especially the even older concept of THAC0, served the same role of reducing required dice result to simulate character growth.
2. Add only half level to untrained checks, rounded down (with that -4 or whatever extra penalty).
This way, those who didn't care on investing proficiencies won't be left out too hard as in PF1, whilst naturally lagging behind somewhat. Although, to prevent lagging too much on the high levels (it's a whopping -9 than expected, if you keep the flat penalty), that -4 untrained penalty should automatically shrink for all creatures on designated levels; I'd prefer 5, 9, 13, and 17, respectively.
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EXTRA. Change the +2 to ASI levels into 1, 5, 9, 13, and 17.
One extra; this one is just an aesthetic preference. I always thought the old 1, 5, 10, 15, and 20 progress look bizarre thanks to the 1st level increase sticking out like a sore thumb; if they switched that "assumed 0th level feat" into an "actual 1st level feat" on PF1's character level table, why not for Ability Score Improvements?