I have wanted to do this for a while. If you were dm and I told you that I wanted to play a plasmid that attached to one of the other players (if that player was okay with it). How would you play the mechanics for it. Sometimes Google shutdown for me so if you guys want to start a private thread with me in it I'll get emails for what you guys talk about. If you don't that's okay with me. Love to hear your ideas.

Yo I love the idea actually, although I am not sure if attaching to another player is your best choice seeing as they would have more power over the body then you would. I feel like Venom would be the more dominant one of the the two but still the idea sounds dope. 

Mechanics wise I'd say just use the Mounted Combat rules and see the player as an independed mount and you as the rider. When it comes to damage you could say that while you're attached to someone you gain the same benefits as the Warding bond spell or you could get some damage reduction ability's like the Interception fighting style or Oath of Redemption aura. You could even give some enhanced abilities to the creature you're attached too. For example if you attach to a creature, both you and the creature share your stats granting both of you each others stat points and letting you use which ever one is the highest. 

Just some idea's, hope it helped! Lemme know if you come up with anything cool :)