Well if my self-imposed shackles are off, and while I have your eyeballs, please allow me to briefly summarize [rubs hands together]:
In the six-figure box set days, this would be pretty straightforward -- the Syndicate plus Spider-Man. A simple removal would then be Hydro-Man, who has had a couple releases, including a thoroughly updated version (much love to Wave 77!!). So the short list:
Beetle and Boomerang are clear choices for 2 of 4 villain slots. I love-love-love that old 80s Beetle armor.
Speed Demon as you mentioned has been released in his animated format only, while his comic costume is distinct enough to merit a detailed update with a new color scheme.
Rhino is hugely popular, and that MiniMate remains in high demand. But also -- the last one was a hybrid version. It had the studded bands of the Syndicate armored version, but lacked the chest armor. A massive win would be a Rhino with parts to alternate looks between "smooth" and "armored." If this came to pass, you would witness a grown man faint.
Spidey of course would round out the box as #5. This is my pitch [insert obsequious bow with hand flourish].
(And just to further muddle things, Shocker was also affiliated with the Syndicate. We haven't gotten a Shocker figure for some time, and the previous version was more 90s-ish than the classic puffy-suit version. So Shocker's also on the table).
But back to regularly scheduled programming: I think I have all the parts for my last PMIF wave?? I picked up a second Sela. Which I will be using for armbands and a necklace to make Nightshade "pop" a bit more. Her costume's pretty plain. And, spoiler, since I'm doing (Cornell) Cottonmouth, I might as well divulge that I have a very absurd but potentially fantastic way to portray his unique facial hair. It's either going to win the whole wave or fail miserably. If the latter, at least I've picked out some snazzy metallic paints to help his green suit stand out. We'll see how quickly I can bang these out. So far, it's... not quickly. 😅