This could be for both encircled troops, or troops in low supply ar''eas. Transport planes should also have the ability to airdrop supplies to troops in need of them. The mechanic worked well in HOI3 and I believe it could work well in HOI4. I don't believe it would be too overpowered since many times, encircled units get killed before they begin to get into critical supply. For the player who doesn't use paras, it would provide a use to an equipment which is usually discarded. By adding this, building transport planes could be a strategic decision when mounting large campaigns. You could have it as an air mission available for transport planes which targets prioritized units (in the division designer) and it would give another advantage to the player with air superiority. This could be for both encircled troops, or troops in low supply areas. Even if you do decide to use paras, your transport planes will be sitting around for the majority of your campaign. After using paras once or twice, I realized that they were only really good for cheese strategies such as paradropping enemy capitals and since they can only be composed of infantry, they lack good combat stats and are only useful in getting extra combat width for port battles. While I love the ability to paradrop, it seems that transport planes are used for only that.

Hearts of iron 4 transport planes