My problem is that each and every house rule plays with how much certain models are worth, more so depending on the scale of the changes they introduce. Changing the turn sequence would screw around dramatically with the value of units, for little appreciable gain. At the end of the day, all you've gained from the switch is a less systematic and more luck-based turn sequence. As it is, the system is set, allowing players to make strategic decisions on their deployment and playing knowing that they or their opponent will get one movement, shooting, and assault phase before they can react. Changing the system would take the irritating uncertainty of the deployment phase, and jam it into every single part of the game. If you can present to me a reason why this is worth this cost, I'll listen. As of yet, though, I've yet to see a single positive advantage to the game itself other than shutting up the people who're too hyperactive to sit around for the unbearably long half an hour of an average turn.
As for overwatch, that's a horrible idea. This addition would make close-combat armies, quite simply, unplayable. Overwatch especially would break any game with a reasonable amount of terrain in half, and turn it into a match of 'sit and wait', oir against combat armies, 'total slaughter'. Again, like the turn sequence, it is only an advantage for those people who cannot think of time as being anything other than perfectly linear. These people, obviously, must have never touched a book, movie, or game plot in their lives.