DND_Reborn
The High Aldwin
I don't know... that is pretty hard to imagine!Ok, another idea (this one may even WORK, can you imagine that?)
Health
HP at level 1: CON + Half HD
HP at higher level: Half-HD per level
Defense
Here's the twist: No attack rolls, all attacks are hit. The only variance is for how much? The attacker rolls its damage roll, the defender roll its HIT DIE + Dex (or Con for Barb, Wis for monks etc) (+prof, maybe?) and reduce the damage from the attack by the same amount. If the attacker has more than 1 attack, they can divide them as they wish or focus on a single target. AC buff from spells or features are added to the damage reduction. Armor's give increased DR. The attack action no longer represent the act of attacking, but the landing of a blow through an opening in the target's defenses. Character are considered attacking continuously during the round.
This way, all attacks are only partial hit and are taxing to the defender, unless they evaded the attack completely by reducing the damage roll to 0.
Skills & Saves
Proficient skills and saves are increased by Proficiency bonus (with your progression scale instead of the PHB's). Expertise gives advantage.

I think I am pretty locked on the CON at level 1 and half HD (or rolling if they prefer) per level after. I don't know if I'd want to grant an extra half HD to starting HP at level 1. Probably won't hurt anything if you did, though.
Defense is an interesting idea. In the thread on healing surges I thought something like spending HD to stop damage might work, but removing the attack roll entirely is really new! But it brings up a lot of questions:
Would it be fun for the players to not roll attacks anymore since their damage is determined and the DM rolls to resist it?
How could you handle the chance of a critical hit?
Would the PC spend more than one HD if they wanted or limit it to one HD per "attack" (i.e. damage source against them)?
Wouldn't it create more rolling (i.e. take longer) since now the attacker is rolling damage and the defender is rolling HD to soak every time?
How much hassle for DM to track monster HP and spent HD for soaking?
I'm sure there are others, those just pop to mind. It might have some merit, but I don't know how well it would play out. What are your thoughts on those questions?