i think that giving garrisons another go would be a mistake personally, there is already a lot of vitriol surrounding it, maybe going for a proper player housing system would be a good idea, or maybe a guild housing allowing you to have trophies on show for boss kills showing the date and which characters were there, like guild achievements.
the main problem with the way garrisons were done is that they were used as an integral system. they attempted to make it interact with everything rather than just giving us the ability to customise it in the way we wanted to, meaning that you needed certain things to help for raid progress as well as the apexis farming and follower table and such.
while nothing will become impossible there will be certain things that become alot more difficult to do, for example, i would prefer the covenant abilities to be unlinked from the covenants i think it would be much better for them to be a secondary talent row or something so that we can swap between them in a rested area,
main reason being that it would allow you to customise and give you much better agency which is one of the key philosophies that blizzard spoke to during blizzcon last year.
the covenants are a potentialy amazing system i just dont like that they are adding some much player power behind them, just make them an aesthetic choice so you can pick whichever one you want instead of feeling forced to take a specific one which is “best”