Now, 4E is not my favorite edition of D&D, but there are some good things that I like in that edition:
1. Healing surges; HD healing in 5E is just clumsy and worse version of healing surges, and I believe it was change for changes sake as not to remind people of 4E.
But why? It was great. It was reliable, while HDs are RNG cluster #$%!.
25% of you max HP is a good amount.
You can have a number of healing surges equal to double your proficiency bonus plus your Con modifier per long rest.
Some 8sub)classes might get a bonus healing surge or two, or more on later levels.
2. Second wind; closely connected to no.1 here is Second wind. Remove it from fighter and give it global usage as a Action. Heal yourself for 25% of max HP, number of times per long rest equal to your prof bonus.
3. Short rest is 5 min. This is self explanatory. 1Hr is NOT a short rest.
4. Attacks opportunity do not spend your "reaction slot", but are on separate once per turn(any players turn) counter.
This would help melee characters keep enemies in melee range or suffer AoO more easily.
5. More things provoke AoO: casting spells that are not melee touch or close blast/cone(without Spell sniper feat), ranged attacks(without Sharpshooter feat), moving more than 10ft within melee range,
6. return to 3 saves or defenses, I'm good with what ever. But it's more fun as a PC to roll all "attacks", so from player perspective all spells could be spell attacks, vs one of 4 defense.
Have Fort save/defense be str+con mod
Reflex save/defense be dex+int mod, and
Will save/defense be wis+cha mod,