Let's just hope the "damage on a miss" stuff stays confined to Themes, as the game development continues. Some of us would like to continue to ignore them.
I don't think it has to be confined to just one feat/theme. But I think (i) it has to be confined to stuff that can easily (trivially!) be swapped in or out, and (ii) be very obvious.I would totaly support the idea of only a very few (or even 1) way to do this, would that make anyone happy?
Substitutabity means there have to be plenty of other options that behave mechancially differently, but occupy something like the same functional and story space (ie Reaper better not be the only way to build your PC as a vicious and deadly combatant).
And obviousness is important, because you don't want people accidentally be making a choice based on labels or flavour and then find that they're stuck with a mechanic that doesn't work the way they want it to.
I think this an argument in favour of a more transparent and clinical style for presenting feat and spell mechanics than the playtest document uses. It's also an argument for something a bit like roles - I'm thinking of the categories used in Essentials to group feats together. It might be helpful if it was easy (via categories, labels etc) to find all the "I kill things dead quick feats", and then easy, when looking through them, to separate the "non-simulationist hp" ones from the "simulationist hp" ones.
I know there are other tradeoffs with going in this stylistic direction - it makes it very obvious that the rules are rules for a game and not just descriptions of PC abilities in ingame terms - and maybe an alternative supporting obviousness is possible.
But as long as it is obvious, and easily ignorable or substitutable, then I don't think it should be a problem if there are one, several, or many.