Celebrim
Legend
PC-style spellcasting is a pain to manage when it's the only thing you're focusing on
PC spellcasting is only a pain to manage when you manage NPC spell lists in the same manner that you'd manage a PC spell caster. Nothing prevents you from selecting one spell of each level to fill all the slots of that spellcaster. In practice, if you simplify spellcasting by giving the NPC some repeatable spell-like effects as attacks, that's very close to what you are doing.
But when we accept that monsters aren't PCs, we can have stat blocks that aren't filled with justifications for other things in the stat blocks. If bugbears are sneaky, awesome, include that. But I don't want to see their hit dice go up so they can justify a higher skill rating or every monster needing a bunch of skills because that's how many skills "the rules say they have."
Again, nothing forces you to fill out the skills of a monster. Moreover, most of the time the system works exactly like you say you want it to work. Bugbears are sneaky and awesome because they have a +4 racial bonus on Move Silently checks. Racial characteristics are a huge fudge factor that basically lets the designer break any rule he wants and do whatever he wants. Rarely do you need the HD to justify an attribute of a monster the way you almost always did in 1e (and would in any simplistic system).
But even more importantly, you are the DM. You are empowered to break the rules. Do you know when I first encountered the notion of a 'minion' monster? It was 1989, and I was reading the ground breaking module 'I3: Pyramid' by probably the finest module writer the game has ever known - Tracy Hickman. In that module, there is a series of encounter with a large number of desert dervishes. As was typical of the monster building rules of the time, Tracy stated up the 'desert dervish' as a 4HD monster under the subheading 'man' (where in the monster manual it could show up along side berserker, bandit, buccaneer, etc.). That made them powerful enemies capable of overcoming PC armor class and thus threatening foes. But he does something incredibly novel in the room containing the largest number of dervishes. Instead of rolling 4d8 to determine the hit points, if you look at the stat block you see something extraordinary - the hitpoints were determined by rolling a d10. By the rules, it's impossible for a 4HD creature to have 1, 2, or 3 hit points. But there they are; 4HD monsters with the hit points of 1st level fighters. At the time there was no way to do that. Now, we'd stat them as 1st level fighters with 16 strength, weapon focus, etc. Or more simply, just 1st level fighters with an additional unexplained +5 bonus to hit. Tracy just considered the problem - "I want a large number of easily killable foes that nonetheless have the means to threaten the PCs" - and created just what he wanted using the toolset where it suitd his purpose and doing his own thing where it didn't.
If Tracy can break the rules to get what he wants, so can you. Every rule for DM's is a guideline. It's not something you should be hidebound to.
What's need isn't fewer options. What is needed is clear discussion of where you end up when you want to just use a monster without worrying about justifying the components of the monster. If those feats go missing, what simple thing can you replace them with to obtain roughly the same level of prowess? Can you build into 'stock monster templates' a default series of generic feat adjustments (in 3e terms, weapon focus, toughness, improved initiatve, iron will, great fortitude, lightning reflexes etc.), so that you end up for a given HD and monster type at about the right level of power using just a simplified stat block? Can you assume average bonuses to AC, attack bonus, and damage without justifying them in detail? I don't see why not. All you need to do is describe them well. I can easily imagine having 200-300 generic stat blocks that let you grab a generic 12 HD abberation or undead or dragon and run it just by flipping open a book and describing the monster to your color. You could do that for 3e, and I think from what I've seen so far you could easily do that for 5e.
But while that would be a great resource, my point is that while you can go from advanced to basic by just ridding yourself of justifcations and trimming away unwanted details, it's not nearly as easy to go from basic monsters to advanced toolkit.