dave2008
Legend
The regular guidelines are probably fine. Changing both the monsters and the encounter rules adds two variables to the equation. Two points of failure. It's probably easier to just increase the potency of monsters to match the power increase of PCs optimised with feats.
My plan would be provide the encounter rules that would work with the monsters as they are. This would require adding more (possiblly a lot more) monsters to meet a "hardcore" threshold. Then provide "hardcore" monsters or versions of monsters that have a higher XP value and allow you to use fewer monsters to achieve the same results. I can't get that level of flexibility by doing one or the other, you need to do both.
5e already shows you can add abilities to monsters without technically increasing their challenge. So it's not even *really* bending the rules.
That really depends. A lot of abilities do indeed affect the CR - there is a whole section in the DMG about how various features affect a monster's CR.
Thinking on what I wrote earlier, alternate abilities - what 4e labelled Monster Themes - that grant reactions and bonus action abilities might work nicely. Few monsters use either because it's hard on DMs. Especially newer ones. But when facing a high level or experienced group, the DM likely has some skills.
More abilities that let monsters do feat style things, like block attacks on allies, take extra attacks, push PCs, and the like.
Those are all great. Just realize that most, if not all, of those abilities do indeed affect the CR in some way. Heck the Orcs "aggressive" trait which only allows it to move as a bonus action affects its CR per the DMG. That is why I think we need another set of "hardcore" monsters.