Shadow pokemon are a feature of gen 3.5 (or just gen 3 game cube games) and are what I think would make a good feature.So first we got how we would aquire them,fairly simple process you put a regular pokemon into a cloning machine after about 20 minutes of waiting bam you got a brand spanking new shadow poke,next we go onto what differentiates them.A shadow clone has Much better ivs and a higher ev gain when leveling
but the downside is it can only use shadow moves,like shadow rush shadow peck etc.
if you've ever played these games you know shadow moves are extremely strong but you suffer a large amount of recoil,next we go to visual design.All that differs them is a Blackish purple aura around them (honestly thats its)
if legendarys are possible to make shadow clones of i suggest somthing similar to the ranch block that they need to have a unqiue floor to transform them,
I only really suggest mythicals to be able to make shadow clones of (so the shadow lugia skin dosent become confusing) unique models maybe should be made for shadow legendarys.
ACTUAL FINAL NOTES
any shadow pokemon can be snagged by someone with a snag machin. (make it a level 2 drop to make it not a obvs reason to have a full shadow team)