I prefer a sleek set of core rules that allow for modular expansion. Overall, I think D&D 5e did this well. I've been able to add on all manner of subsystems, classes, spells, and items from subsequent official books, TPPs, and homebrew without breaking the game. I do wish they spend more time steamlining and rules/advice for adding to the game. The DMG material for alternate rules, creating monsters, developing encounters, creating magic items, etc. doesn't feel as well designed as the PHB. I'm hoping that the 2024/5 DMG takes the lessons learned over the past decade, and the more time and money they've had to redesign it, and give DMs and TPPs much better tools for adding to and altering the game in ways that don't ruin game balance--or at least make it easier to make informed game-"breaking" changes and additions.
Also, while the core system is pretty darn sleek and robust, the presentation makes learning and referencing the rules much more complicated than necessary. Here is to hoping that the new PHB and DMG improve their layout, organization, cross-referencing etc.