Alright so these are just a few Spidey characters, mostly villains, that I've fancast for the MCU. My guess would be that many of these actors will be considered too old, but its a fan cast so let's just say you can imagine them in the role at the exact age you think would be appropriate, because it's not like any of this is likely to happen. Which is why I considered suggesting Robin Williams for Doc Ock but it made me sad to think about it so I chose someone that's still around. Anyway, I hope you find a suggestion that inspires your imagination, until we get to see what Marvel really does with these guys.
I'm not great with inserting photos so you may have to do some googling if you're unfamiliar with some of these folk.
I haven't cast Harry or Uncle Ben simply because I don't think I have enough of an idea of the direction they're going with to choose actors that would fit this universe.
Mary Jane- Olivia Cooke
Her turns in Bates Motel and Me, Earl and the Dying Girl were fantastic, and she's about the right age, and is all set on the hair colour. One of those formal wear girls Spidey was carrying in recent set photos also looks incredibly similar to Ms. Cooke, although that may just be me dreaming...
JJ Jameson- Nick Offerman
Ron Goddamn Swanson, people.
Betty Bryant- Mary Elizabeth Winstead
Since her fantastic work in recent films like 10 Cloverfield Lane and Faults, this role is far below her in all honesty, but I had to have her in the MCU and she is a little too old to play Gwen or Black Cat to Holland's young Spidey.
Vulture- Christipher Waltz
I know Keaton is playing the role but it hasn't been confirmed as of yet and that makes it fair game.
Goblin- Mel Gibson
Well we all know he can play batshit crazy. After his recent return to action in 'Blood Father', Gibson showed he still has that excellent range, and can play a straight man and a loony very well.
Doc Ock- Peter Dinklage
I think this sells itself.
Black Cat- Alison Brie
Alison won many hearts in Community, but she has a bad side and a delightful playfullness that I think would shine here.
Kraven- Dwayne Johnson
He looks the part, but more importantly, he can play pure drive and animalistic rage, as seen in 'Faster'.
Scorpion- Colin Farrell
Inspired by his smarmy performance from 'Fright Night'.
Mysterio-Kevin Kline
I see Mysterio working best with a comedic angle, playing a formidible but second tier villain. Kline is hilarious at over-the-top grandstanding, and I can just see him putting on a show and puffing his chest inside that suit.
Rhino- Nathan Fillion
If you haven't seen him as Captain Hammer, don't worry, he's got this. Would love to have cast him as Carnage based off his terrifying performance as Caleb the psychopathic priest in Buffy, but I think he's lost that menace as he wasted his time aging on Castle. What a waste of a great actor's best years...
Venom- Simon Baker
Imagine him younger if you must. He comes across very confident, intelligent and strong, but can also be vulnerable for the times when we need to see why he is giving in to the symbiote and its effects on him.
Carnage- Jeffrey Dean Morgan
Negan. Also played a demon for a short while on Supernatural and was very good. Ruthless, cunning, and most of all, gleeful in his evil when he plays those roles.
Electro- Timothy Olyphant
I dunno. TASM2 kinda ruined this character for me, but Olyphant could pull it off.
Sandman- Liam Neeson
I don't love this character, but I think Neeson could make him interesting.
Lizard-Geoffrey Rush
Rush plays a great gentle presence that is easy to imagine in a mentor and scientist role. But he can add great menace to his voice, like with Captain Barbosa, for example.
Morbius- Jeremy Irons
Speaks for itself.