I'm not entirely sure either, since as mentioned in the OP, Maokai's biggest change in the Tank Update took an ability that was unique, if a little troublesome to balance, and made it into something much more generic. In addition, it took a tank that I think innately works as a warden (his E and old ult are both defensive abilities by nature), and tried to turn him into a vanguard, with mixed results. I don't think any of the updated tanks were really more specialized, and in fact I think the Tank Update actually made the tank class much less diverse overall, while also causing them to overlap even further with fighters. What disappointed me with Maokai in particular was that the rework he received previously had done an amazing job of making him feel much better to play, while still keeping him fair overall. It feels like much of that progress was undone for no real reason after the roster update hit Live.
Personally, if Maokai is to change again (and I think that will inevitably happen), I'd rather bring him back into warden space. Personally, I always imagined Maokai as a defensive spell eater, as opposed to Galio, who's more of an offensive anti-spell Warden (or was, at least, since now he's more of a diver/vanguard). All of his old spells served as a means of intercepting incoming threats, and that I think should be his defining characteristic: at this best, Maokai should be this near-impenetrable wall who can categorically stop lethal spells by absorbing them himself, and by slowing down the enemy's advance. In order to achieve this, though, he should take the time to set up a perimeter via saplings, and should always keep a close eye on his positioning relative to his opponents and his allies. This alone could already be unique enough to set him apart from other Wardens, many of whom are much quicker and capable of providing immediate relief to their allies, though can typically only focus on one opponent at a time.
In order to allow this, I'd remove Maokai's current ult, and replace it with a more intense version of his old ult, one that would completely redirect all spells in an area to target and damage Maokai instead: it would allow him to protect his allies, and would synergize with his passive very well, but would also force him to rely much more on his sustain in order to survive teamfights, which could allow his current innate to be nerfed early and buffed late.
