If you're interested in the work we did to match up 5e and the setting of Symbaroum, the free primer will give you some ideas:
Ruins of Symbaroum [5E] - The Promised Land - Free League Publishing | Symbaroum | DriveThruRPG.com
To answer things as simply as possible, some folks will want to make use of our classes outside of the Symbaroum setting. Other folks might want to import a class from another 5e compatible product into the Symbaroum setting. So a completely different hit point progression scale doesn't make a lot of sense. But what we can do is tie Hit Dice to origin and size — in other words, all Medium origins have a D8, and any Small origins have a D6. That helps scale things back, since there are no D10 or D12 Hit Dice characters (even Ogres and Trolls, though at the edge of Medium size, don't count as Large).
We also completely change the rest mechanics, introducing an extended rest. Long rests no longer restore hit points or Hit Dice (you need an extended rest for that). So a higher level character with a higher number of hit points has a higher number of resources (hp and HD) when they enter the forest. They can last longer and get deeper in (which makes sense, they're more experienced). But they will reach a point where they don't have any more HD (and Hit Dice in Ruins of Symbaroum represent both your ability to recover hit points and deal with Corruption so even healing magic is limited) and will be just as vulnerable as lower level characters.
Also, monsters/fights in Ruins of Symbaroum are much more likely to be unbalanced — there are places in the forest that are extraordinarily dangerous and you should have no expectation of a 'fair fight' if you start poking into such locations. Monsters also have the ability to deliver much more than just hp damage — we make use of multiple scales (including Corruption, exhaustion and conditions) to represent the characters' health and overall situation.
The Kickstarter is ongoing and we have a lot of updates and questions and answers there:
Ruins of Symbaroum for 5E