Sure... but that's adding more homebrew to balance the breaks that your homebrew created... sound familiar? ^.^
In seriousness, tables can always run the way they want to, but most rules are designed in specific way and for good reasons (there are exceptions - I'm no sycophant and there are certain rules that I've been vocal about my disagreement with ^.^), and the initiative lock is one of those. There are quite a number of ways that allowing people slide around in the initiative order can break things or be exploited in unbalanced ways. Here's another one that you need to book-keep: Player: "I'm going to delay my turn until after the monster goes, I want to see where it moves to so I can place my spell better.", DM: "The monster delays its turn to go after the players, so it can decide who to attack", Player: "Er... so... who goes?" You can make another caveat rule for that case, too, if you want, but it's far less of a headache if you just use the ready action as originally written, or, if you need to, expand how powerful the ready action is (allowing the ready action to be a full normal action, unlimited by other restrictions, for example, is a common concession that doesn't break anything, or unbalance things as long as you also nix the need for concentrating on a 'ready' at the same time, and move the 'casting' moment to the release of the ready, for the purposes of other reactions).
