I had a look at it today and fixed it up a bit. I think it's a really great engine (didn't I suggest it back in the day ?) but one thing it's lacking is a way to customize the regen/decay to the personal needs of a race.

Right now you set the fHPRegen or fHPDecay buff on a player and everything just works. That's great. But the problem I have with this, is that there is no way to further customize this behavior while still benefitting from the Engine doing the regeneration/decay for me. For example I have a Medic race on TF2 that has a healing wave and receives uber for the HP healed around him. I'd love to use the regen engine to do the HP regeneration for me, while still being able to give the Medic uber.
Same thing with the decay "buff". Soul Reaper gives a decay buff to enemys around him, meaning they'll lose HP, but the Soul Reaper is never credited for it, as there is currently no way to tell who invoked the decay buff on the player.

Does anybody have an idea how we could improve upon this engine? Maybe brush it up to be similiar with the aura engine?

even the aura engine(while its ultimately powerfull) is lacking some features. for example you cannot get the owner of the aura you just entered.

idk if sourcepawn supports pre-processor macros with arguments(i think it does) why don't use them

at feet cuz i feel it looks weird popping out of heads

I feel like it looks weird/buggy when it pops from the feet, which is why I felt like fixing it due to it looking buggy to me. I also agree with Diablo that feet are an obvious spot to put particle effects, TEs and other stuff to indicate movement skills etc.

Having played with the decay at the top for a bit now I really feel like it fits better there.

I don't really have an idea how to handle this yet