Sounds sensible on the face of it but in a one-off game it's even more likely that you could get a freak outcome. Two legs would usually favour the stronger side - in the same way that underdogs that end up with a replay in a cup tie usually find that they've 'had their chance' to win it first...
With the difference in resources between the Premier League and the Championship, I think that would be almost pointless. Once you start delving down as 7th in the Championship and as high as 17th in PL, you're hardly ever going to get a new team take the PL place.
We need the play-offs as for most clubs the season gets tedious otherwise. Best suggestion I've heard to revamp things is far simpler than all of the different formats. Make the 4th, 5th and 6th clubs fight for the right to a play-off. In other words, set a threshold beyond which the 3rd placed...