Simple solution to horizontal progression. Every new raid release you stat squish all currently owned old raid gear down to equivalent of level 60 pre-raid dungeon gear. Players can then either play new raids to get new sets/items to raise their old stat squished gear back to raid power level, or play the old raids again to get back to raid power level/collect missed pieces. Maybe cap players to 2 raids a week or weekly raid rotation when you have many raids out just to slow re-gearing down. All raids same difficulty, all gear relevant. 1-60 experience will still play into raid content the exact same as fresh release and can come back to the game after a break at any time to jump right back in to all content. Could even do similar with new PVP sets. My two cents at least.