In my view a chess opening is characterized by a typical pawn structure and plans associated with it. As such a single move like 1. e3 should not deserve to be called an "opening' because there are no ideas behind it besides that it allows you to develop queen and bishop. True it can transpose to somewhat decent openings like the Colle system, reversed Stonewall, reversed French and the English, but it is impossible to give you a single "idea" that is behind all these openings. Similarly you would not get a good answer asking for the "ideas" in 1.e4 openings, because it can turn into so many different openings each with its own ideas. Call it "too broad" if you prefer.
OK, 1. e3 is not the worst first move you can make, but it is unambitious, basically giving black free choice in the final pawn structure in the center.
In my opinion the only "ideas" behind 1. e3 are not found on the board, but rather in
- getting the opponent out of book, and
It is nothing that black should fear and does not require any preparation for black either, as you can just play whatever setup you want against it.
As for the Keoni-Hiva Gambit and similar suspicious lines, I don't see how playing f4 helps white's development. Perhaps if black gets greedy and starts eating pawns (fxe3, exd2) white can get a lead in development but black is not forced to do so and just playing normally I'd be happy to face the Keoni-Hiva Gambit with black.