Top 10 Back to the Future characters/variants I'd like to see:

 By the way things are going, I might have to create a NECA tag... but speaking seriously, Back to the Future as a toy line requires multiple variants of the same characters. Like we have 1985 Marty 1955 Doc Brown 1955 Biff Tannen but we don't have a 1985 Doc Brown a 1985 Biff Tannen or a 1955 "Calvin Klein"...or even an Enchantment Under the Sea Marty with vanishing hand. The vanishing hand was added to another 1985 Marty. Hell we even have a 2015 Marty McFly disguised as Marty Junior, but not a 2015 Doc Brown. Hell, in the intro I already mentioned a few variants that I want...

Sadly this list is 100% George McFly free due to the legal nightmare involving Crispin Glover's likeness rights. To be fair I can understand why mr. Glover would say no especially if it's related to Back to the Future. Also I need to point out that I'll try to keep the Doc and Marty variants to a minimum. There might be around 4 figures total that are Doc and Marty variants.

10: 2015 Doc Brown:

We have a BTTFII Marty, so a 2015 Doc makes sense.

9: Enchantment under the sea Marty:

Since we can't have George McFly (and Lorraine, since  wouldn't make sense to have her without George) then, this Marty covers the iconic scene where he nearly fades out of existence.

8: Jennifer Parker:

Yes he should have a Claudia Wells and an Elisabeth Shue head. It would be a really dick move to release both Jennifer's as separate characters...

7: 1985 (Dark timeline) The Biff (with "enhanced" Lorraine Baines Tannen):

 Since Biff killed George McFly in this timeline, Lorraine is doable as a figure.

I know I'm "cheating" on my list, but while The Biff could Sail on his own comet Lorraine wouldn't... so, a two-pack makes sense here.

6: Griff Tannen:

He has a Pitbull Hoverboard, period. But it's all about having "balance" on the roster. While Old Man Biff Tannen is the true villain of the story, the Tannen that Marty actually "fought" in 2015 ia Griff.

5: 2015 Old Man Biff:

Like I said with Griff, it's all about balancing the roster and Biff Tannen is the main villain of most of the fire therefore he's needed in as many forms as possible.

4: Principal Strickland:

There is an ulterior motive for having this character on my list. Part of me also wants to diagnose 1985 version so I could mix and match accessories to have my secret character on a different display.

3: Mad Dog Tannen vs. "Clint Eastwood":

Once again I'm cheating I know I know but it makes sense to have these two characters in a two pack, since BTTF3 is a Western and these two had a duel.

2: 1885 Doc Brown and Clara Clayton:

Another 2 pack!? I know it kind of feels like I'm cheating... the reason I'm pretty much putting everything back to the future 3 in Two-packs just to get it done quickly. I understand that the majority of people are not very fond of part 3. Also I'm not too sure that a Clara Clayton figure would sell well on its own.

1: 1985 Doc Brown  (Twin Pines Mall):

We have the 1985 Marty with the camcorder to record the time machine's first travel. So we need a Doc Brown figure to go with that Marty.

And for those of you worried about breaking balance there is the already mentioned enchantment Under the Sea Marty to pair off with the 1955 Doc, or the currently existing audition Marty. 

Honorable mentions:

Howdy Doody Marty, BTTF2 Dark Timeline Marty,  2015 Marty, Needles, 1955 Biff's gang, Griff's gang, and last but not least: Manure Biff.