My biggest gripe with 5E and most other editions is that the trope "terrifying arcane power gained by unearthing of dark, ancient secrets" - which is central to so many published campaigns (perhaps the granddaddy of tropes) and implicit in a lot of the descriptive text - is not reflected in the mechanics, like at all.
There are no spells, feats, class features, magic items etc. that can be more easily (or only) gained through nefarious sacrifice rather than plucky ambition. There are no moral trade-offs required for the upper echelons of arcane or divine power. 8th and 9th level spells don't have any profound costs, just gp equivalents. And the "ancient secrets" are almost always re-skins of existing knowledge or an adequately CR balanced magic item.
It feels like a very central trope to fantasy adventure is just artifice and macgunnfin-ism in D&D.