So, the thing about the existing templates is that they are all very simple changes that--and here's the important part--do not alter the creature's CR.
It's not simple to state how a template would alter CR in 5E, because CR is based on a lot of numbers that simply don't translate to a standard template. Like, average damage over three rounds? That's going to be a much more dramatic change between CR 3 and 4 than between CR 15 and 16. A single +1 CR template can't accommodate both.
I'm not necessarily saying it can't be done, but I think it would be a lot harder and more complex than people realize, and it would still require some math and calculations on the DM's part--which, in many cases, would make the template no simpler than just making alterations by the monster creation rules.