D&D 5E Homebrew martial archetype: Planewalker Warrior

[Quickleaf]

I'm attempting to convert an old AD&D kit called the Planewalker Warrior (from The Planewalker's Handbook by Monte Cook) as a 5e fighter sub-class. This is for a conversion of my friend's 15th level character from our old Planescape campaign. He'd taken this kit in our game, and when I couldn't find any 5e martial archetype that was a close approximation, I decided to try and convert the kit. Hopefully we'll get to play this year & I'd also like to get a nice customized illustrated character sheet as b-day gift for him.

Part of the challenge here is stretching the concept to fit the 5e martial archetype with its features at 3rd, 7th, 10th, 15th, and 18th.

Your feedback is welcome!

Spoiler: Flavor text of the AD&D kit

Planewalker warriors are experienced fighters who know that bashing someone with an axe doesn't solve every problem. They’ve heard tales and seen firsthand what the multiverse hold, and they know they can’t defeat everything that comes along. There’s always somebody, or something, tougher out there. Nevertheless, they have the skills and the style – and the knowledge – that enables them to get out of virtually any scrape.

The AD&D Kit:

• You were required to have the Planar Sense proficiency in order to qualify for the kit. Planar Sense allowed you to predict imminent changes in a plane's breathability, temperature, gravity, ground stability, and so on.
• It granted you the ability to more easily hit monsters only hit by +X magical weapons, such that you instead only required a +X-1 magic weapon.
• It also granted you a bonus proficiency called Planar Survival (basically the 5e Survival skill with a planar re-skin).
• You were restricted from taking proficiencies not immediately useful to adventuring.
• Suggested proficiencies included Direction Sense, Portal Feel, Planar Survival, and Survival.

Notes on converting to 5e:

• We played it as a kit that was more about awareness, exploiting knowledge, and cunning, than brute strength (even though his PC was very strong).
• The two 3rd level features from the Horizon Walker ranger – Detect Portal and Planar Warrior – are a good fit, and going to borrow those (perhaps with slight tweak).
• The 7th level Battle Master feature – Know Thy Enemy – is exactly the sort of character this subclass seeks to emulate. Thinking of borrowing it outright.
• I'm thinking adapting the "Planar Sense" proficiency to a 10th level feature. I'm using the Eldritch Knight's 10th level Eldritch Strike as a rough power balancing gauge. Maybe something like "considered natively adapted to all environment"; for example, this would mean no disadvantage on any attacks underwater. Not sure if that's quite enough though..
• For 15th level, I'm considering adapting the rogue's Blindsense 14th level feature and/or the Inquisitive rogue's 3rd level Eye for Detail. Something where the character isn't forced to chose between attacking (or taking Action) and assessing the situation / locating an invisible enemy. Still not sure about this however...
• For 18th level, I'm still brainstorming... thinking something based on Cavalier's Vigilant Defender, but instead of using the special bonus reaction for opp. attacks, might be "any subsequent attacks made by the same creature against you before the end of its turn have disadvantage on the attack roll" (i.e. Multiattack counter), or "Gain Truesight until the end of that creature’s turn", or "Move 5 feet without provoking opportunity attacks." Still not sure about this...