Personally, I'm waiting until I see the final text in order to decide which homebrew rules I'll be implementing. Based on what I'm aware of now? I'll likely shift some of the hunter's mark related features around. I personally like the idea of rangers getting concentration removed from hunter's mark somewhere around 5 to 7th level. It would make the future upgrades feel less top heavy and still allow the idea of that spell being a signature thing for rangers. I don't actually hate the idea of Hunter's mark being a signature ability of rangers in theory, I merely dislike the execution.
Additionally, I think part of the capstone should be the ability to cast Hunter's Mark at will without expending a spell slot. If they are really so insistent on making the spell so vital to rangers then it is a logical part of the capstone to include. I frankly don't even think that a ranger having perma-advantage on one target at a time is hardly game breaking at a level where players are literally leveling towns and rewriting reality.
I am, however, hesitant to alter the spell itself to do something like remove concentration or scale the duration both because I don't want the spell to literally be the same as hex, and I also think hunter's mark being "upgraded" should be a ranger's "thing" if anything else.