It is far from a foregone conclusion that being able to cherry pick is necessarily stronger than having a prepackaged set of abilities (see the Slayer fighter vs. the core fighter, or the Thief rogue vs. the core rogue for examples of this done well).
And 4E Slayer vs. PHB fighter is exactly what I'm advocating for here, and precisely what we are not getting based on this Ro3.
A 4E Slayer is
not a prepackaged version of the PHB Fighter. He gets many things a PHB Fighter cannot. A PHB Fighter cannot duplicate the Slayer by chosing feats or powers. A Slayer is simple, but different, from the PHB Fighter. And this enables it to be both simpler, and balanced. The Slayer is capable of doing things the PHB Fighter cannot, and vice versa.
On the contrary, the current design of 5E's Themes is simply a prepacked set of feats. The custom-feats builds can exactly duplicate the Theme, plus they can combine whatever best features of various Themes they want.
This means that a Theme-based character
cannot be better than the custom-feats character, unless the custom-feats character is simply poorly built. In a game that allows custom-feats, there is no real reason to play a Theme unless you are new and don't know better, or are just plain apathetic about building characters effectively. On the contrary, a 4E Slayer can easily appeal to a player who wants to build an effective character, and is a viable competitor with the PHB Fighter.
I don't know what "constraint" you are seeing on feats that doesn't apply to "features."
"Feat." "Feature." "Power." "Spell." "Ability." "Maneuver."
These are pretty much identical terms right now in terms of 5e development.
A feat must be designed to be able to be taken in combination with all other feats, and to be roughly equivalent in power to all other feats.
A Theme Feature would be attached to a specific Theme, and require taking that Theme to get it, reducing the possible combinations with other things. You can put in features that are as powerful as multiple feats, or that are less powerful than a feat, but put together, add up to a worthwhile Theme.
It's a very different design space. There is much more freedom in designing Theme Features.
Hence my comment that if you let 4e characters spend a feat and get a theme ability, it would be basically the same thing.
No it wouldn't be. The 4E Themes are additional options, parallel to feats. You get one, basically for free. Spending a feat to pick up theme abilities means giving up a feat, to get something you could have gotten for free by taking a theme. It might be worthwhile, might not, but the salient point is that spending the feat is not the same as taking the theme. The theme is still very much worth taking, even with the existence of that feat.
The 5E Themes are
in place of feats. This is very different. The Theme costs your feats. And gives you exactly what you could have gotten by spending those feats by hand. Or less, since it's highly likely that better combinations of feats will exist other than the ones the devs put together.