I like this.Level 1: Movement speed halved
Level 2: Hit Point Maximum halved
Level 3: Disadvantage on attack rolls, saving throws, and STR ability checks
Level 4: Disadvantage on all other ability checks
Level 5: Movement speed zero
Level 6: Death
I might add in:
1) When reduced to 0 HP, you are knocked pone, suffer a level of exhaustion, and are down. While down you cannot maintain concentration, you cannot take reactions or bonus actions, and you have disadvantage on all attribute checks, attacks, and saves and cannot gain advantage on them.
(Downed foes are obviously a lesser threat; some enemies will finish you off, others will move onto bigger more immediate threats).
2) If you take damage while down or fail a death saving throw, you suffer a level of exhaustion. If you suffer a critical hit, you suffer 2 levels of exhaustion instead.
3) While down, at the end of your turn you must make a death saving throw. You can expend a HD to gain a bonus on this saving throw equal to the amount rolled, and may choose to expend the HD after you roll. On a 1-9 you become unconscious and take a level of exhaustion (and remain down), on a 20+ you regain 1 HP. If you roll 10+ 3 times, you stabilize and no longer have to make death saves.
Anything that raises your HP above 0 removes the down condition (and unconscious usually).
Next, use Gritty rests (so a long rest is overnight), and allow PCs to expend a HD to recover a level of exhaustion over a short rest.
I also tie expending HD to magical healing (whenever you are healed, you must also spend a HD and regain those additional HP. If you have no HD, magical healing can only stabilize you. This also applies to potions. (Cure Wounds has the bonus that you can replace the d8s with whatever HD you expended). Exceptions are: regeneration spell, ring of regeneration, heal and mass heal spells, power word heal). To balance this, at the end of a short rest roll all expended HD; on an even value 4 or higher, that HD is regained.
(This is based off of 4e's healing surges; magical healing enhances your own healing abilities, it doesn't replace them, unless extremely powerful.)
Note that someone reduced to 0 HP does not immediately become killed if someone attacks twice. Their AC is normal. Enemies have advantage on attacking you.
Tank types are better at getting up from being down, because of their larger HD.