Dude - I was trying to work on a "T&T Quest" game that merged Tunnels & Trolls with WHQ. Only problem is that T&T combat system isn't really suitable for tile-based miniature play (it's one big melee). I started to work on a system where you use small dungeon tiles and move one "room" at a time, like their solo adventure books.
I eventually came up with the idea of making it more like DungeonQuest where you could either go solo or have up to four delvers competing/co-operating.
Hope that's given you some food for thought anyway. I might pick it up again one day myself :)
Remember, sometimes you roll a one...
I think I was working on a similar idea at one point. :) At the most basic level, the gold rating of the monsters can be converted to Monster Rating with a little dividing, and I'd have a Pedlar/Market event to let the characters go shopping in the middle of an adventure. Solo character play is an interesting challenge (especially when traps are based on selecting a character in the party - Id's substitue that with DEX or Luck L1 SR) , but then T&T can be pretty brutal anyhow.
Your point about the rooms not having grids and just triggering encounters is pretty much how Dungeon Run works - but the play is less co-operative (I don't feel compelled to play it at all). In fact modern good-looking crawler games where the party works together seem to be a rarity these days.
The grid can still be used for movement (SPD etc) and perhaps to define whether or not a character is in hand-to-hand combat or missile/casting (i.e. more than one square is between the monster and hero) - because in WHQ it's usually only a turn of so before characters get totally surrounded (pinned) by monsters closing in. But yes, one of the attractions to T&T is the lack of needing grids - or even figures! But there's something so wonderfully tactile and inspiring about heavy floor tiles and miniatures...
The problem is finding players who understand both T&T and have WHQ set! :D Although AHQ might be a good contender too.