There is nothing much to chose between England, Wales and Ireland, but a massive gulf to the others. Scotland sometimes mug a good team at home, at the rate of one every two to three years, but away they are not winning at the big three.
Given the rate of 6n grand slams, it is time for home and away, top 3 to compete. Then on the last weekend, post title, a play off double header- Div 1 to 2, and two to the next tier down.
Thus yo-yo ing will be frequent, annual trips will not happen, but that actual Grand Slam will mean a hell of a lot, the rugby will be great.
Look at this year's 6n- Eng have to win away in Cardiff and Dublin. That is is, the home games are a conclusion- ITA have never beaten England anywhere, and France once in the 6n at HQ, Scotland never in the 6n at HQ (last win 1984). Ditto Wales only have to beat Eng/Ire in Cardiff, and avoid the banana skin in Scotland, which they do most times. Could Wales beat both Eng and Ire away? No chance. Ireland in Cardiff and London? Don't think so. England- no chance- win one away maybe, as they did this year. Grand slam- once ever ten years- far more reasonable and only done by a top, top world class team. Every game matters, no more flat ITA at home weekends, and instant revenge available after a loss as it is home and away. One fewer 6n matches for the champions and runners up in Div 1, whilst the second tier has its first and third teams having an extra match to bring it up to the current level of five.