I think Arnim Zola returning could be pushing it.
His appearance in TFA didn't set him up for anything obvious, so his appearance TWS was a surprise, not the least to Cap.
Then it looks like he died. If he comes back again, it could look tacky.
I actually feel like we will and should see Zola, in his traditional comic form at some point. His appearance in TFA actually does set up/foreshadow/plant the seeds for his transformation into Robo-Zola. The blueprint for the Robot is in TFA, it's seen when Zola grabs it and stuffs it into his case as he's evacuating the factory facility. Robo-Zola actually featured in the Super-Soldier tie-in video game but that has been rendered seemingly non-canon by Von Strucker's appearance in TWS and soon to be AoU.
Then there's Zola's presence in TWS. I was aware that Zola was gonna be in TWS somewhat early on and a part of me was kinda hoping we would see Robo-Zola as the twist villain, big bad but for a while I was expecting his appearance to be more of a flashback to the WW2 time so the fact that what we got was somewhere in between was pleasantly surprising for me. The writers actually said that they scripted a bit where you see the Robo-Zola get up and walk away either right before or after the bunker is destroyed so they were seriously toying with the idea. Pierce says "they cost ME Zola" and seeing the modern USB port plugged into the '70s style computer system implies that significant work was done and it's possible and very likely that a digital copy or several of Zola's brain are still in existence, just that Pierce and his wing of HYDRA no longer have direct access.
It's like an equation. We have two seeds. 1(Robot blueprint/TFA) + 1(Zola's digital brain/TWS) = 2(A seed for a potential future appearance of Robo-Zola). Even in the scene in the Bunker, Computer-Zola repeats the HYDRA motto, "cut off one head, two more shall take it's place" complete with animated graphic showing the HYDRA emblem dividing like a cell. It seems like it could be some more foreshadowing. I guess a question is when might we see Robo-Zola? Personally I would be pleasantly surprised to see him as a secondary/sub-villain/henchman in AoU. We already know that HYDRA will have a role in the story and will likely be dealt with decisively in AoU. Cap's fought many a mechanized villain in the comics i.e Zola, Machinesmith, Ameridriod. Now that I think about it, Zola's also the main reason why Bucky is the way he is now, that's something I could see being worked into Cap3 possibly.
For Cap3, yeah I would like to see some new villains. I think I might have posted in this thread previously. Zemo would be cool but if they use him they should reveal that he's serving/working for the Red Skull, much like he was in the comics. Even in Bloodstone Hunt [blackout] Red Skull was revealed to be secretly shadowing the events via his agent/henchman Crossbones(his first comicbook appearance)[/blackout] If they bring back Red Skull could they do it without HYDRA? The writers already said they may want to take a break from HYDRA. I think the best way to go would be to bring AIM into the mix perhaps. Maybe HYDRA sold them or they acquired a copy of Zola also? The writers mentioned '50s Psycho Cap before backpeddling on that and I think they've mention ideas for both Zemo and MODOK. If they do decide to bring MODOK into the MCU it should be in a Cap film. MODOK to me feels like one of Cap's arch-villains at least from the '60s/'70s, infact I think MODOK first appeared as a Cap villain. Bucky is gonna be in the mix but I don't he should used as a villain again, maybe make Cap and the audience question it for a bit but have him be revealed as an ally again. Imagine how cool it would be to see Cap and Bucky teaming up and taking on a MODOC squad or having to put down Robo-Zola once and for all? I can see Dr. Faustus being in the mix at some point as well, they could do some psychological thriller type stuff.
Either way the creative team has a lot of options. After TWS I got a good feeling about Cap3.