With that much arbitrarium, you might as well be playing a videogame. I want my games to be internally consistent.
With that approach to design the consistency is with
level and xp as opposed to
stats and feats. Really all the stats, feats and other fiddly bits are just proxies for level in any case. There's no actual self-consistency, just a lot of detail. Sometimes the detail is fun. I used to love adding templates to monsters in 3E.
However, there's no build consistency for monster traits - they simply exist, bolted on to the monster "chassis" in order to make the monster behave in some way like the fluff.
For instance, monstrous centipedes have a poison attack. This is not a "level 1 vermin poison feat" that they purchase from some kind of build system. It's not an ability that a PC can buy as a background, class feature or specialty. It's arbitrarily chosen as an ability that the designers think will be an appropriate attack for the level and xp assigned to the monster (or vice-versa, it's designed, then some figure is pulled out ogf the air as to what level and xp it is worth)
I think one 5E "monster design" goal is to be able to build monsters either way round. If they do this correctly, that will mean two things - amongst many:
1) Traits such as poison bite attack will need to have a self-consistent build system (at the very least, well-written guidelines)
2) There will be target numbers for what constitutes as suitable threat for a level A xp B creature. Attack bonus, hit points, AC, number of "special feature build points" or however they want to manage it.
. . . and you should be able to use either or both systems as a guide to building cool challenges for the PCs. You can go straight to the numbers, 4E style (gets my vote), or you can build up from components 3E style (looks like this will get your vote).
I think this is a tall order for the designers, but look forward to seeing it when published!