I've always liked the idea of having more healers in TF2, because if you're playing 6s, you have no one to heal you when your Medic dies, it's only you and medpacks, maybe Concheror', but that's not direct healing.
They can effectively make Soldier a 2nd healer in TF2, by buffing Concheror even further, give it a passive AoE healing to all nearby teammates and use the same mechanic, by not taking damage for X sec you get healed up to 4 hp/s. In order to make this weapon not too OP, make the required amount of damage needed to charge the banner by ~50%. Also Medic's self healing + Banner AoE should not stack or force Medic to receive only +1 hp/s max from it.
This will effectively make Soldier the Lucio of TF2, the class that can provide AoE healing and speed boost. (sorry for OW reference lul)
Scout can also be a healer with the Mad Milk, but the problem of that weapon is that it takes way too long for it to recharge, so I'd say make recharge 10 Seconds but nerf healing to 50% or 35% instead of 60%.
The Engineer and Mini-Dispenser idea was made up long time ago, which focused on making Engineer a more of a Support class, by giving him Mini-Dispenser that deploys faster, can't be upgraded, gives less Ammo/Health/Metal as well as Mini-Teleporter, deploys faster but can't be upgraded, works like level 1 teleporter and has limited uses.
In order to make Heavy a bit of a healer, give him 3 Dalokohs bars, he can consume it only once every 30 seconds but can throw it 3 times to make small healthpacks, and every single bar recharges in 10 seconds