Here's my proposal for the icons (edited to include "Quartic").

For Constante, Linear, and Bezier I used the "In and Out" mode and did 2 versions for Bezier, because I think it is the only one that allows editing, if so it should indicates this.
From Sine to Circular I used the "In" mode because I think it's easier to see the differences. I tried to use the colors to suggest the dramaticity of each curve, beyond the curve itself.
For Bounce, Elastic, and Back I used a violet gradient to differ from the others.

Please let me know if this they are enough and appropriated, or if there any changes I could do to improve them.
Inkscape SVG:

Looks great IMO :)

I think the second Bezier one communicates the editable nature better

Great icons overall!

And I got the same feeling for the second one! I would lighten up the handles a bit, they are now almost invisible and make me think in combination with the bright ends (at least in my mind) that I'm looking at the percent symbol %.

For the ease-icons: Don't you think the red misleads the user? Red is reserved for fast motions but in our case the opposite is the case - it slows down on the red ends.. what do you think..?

Greetings and thanks so far!
Thomas

Oh, I just noticed that the quartic (4) icon seems to be missing from the svg

I've tried adding these to the blender_icons.svg from master

Hi, thanks for the nice comments. I agree to the issues and I tried to solve them.

Easing icons only:

I feel something different from the other icons, maybe it's the black outline and I'll try to improve it soon.Let me know if there are something more to do.

Hi @Paulo José Oliveira Amaro (pauloup),
I like the new ones very much! The ease-icons are much more clear now and the bezier handles are more prominent and distinctive. So in my opinion, it's good to go.

The style of the default icons is much nearer to the tango icon guidelines style than to the "rainbow" styled icons you created here (no offense, I like them very much) ... but I think that they fit the purpose very much! I like especially that you symbolized the incline of the curves via the colors (from green to red).

Greetings, Thomas

Thank you Thomas!

I made some changes, like adding some color to outline (just a little) and bright up some colors.
In my opinion, now they match the existing icons enough, beside the color differences. So that's the final version.

I'm not sure that having the ease icons and the handles on the bezier fading to black read that well, especially in many of the dark themes that people use. Having it white (as in the first version) seems closer to the style used in the other icons (e.g. modifiers - M16 or M25)

You're right about icons M16 and M25, they have a lightgrey degrade to blue. The easing icons could keep this style, but it actually decreases contrast between the affected part and the rest of the curve. It seems to me the darkgrey / blue curve works better, so in this new proposal (v4) I've just bright them a little. But I can change it if you still think different.

Comparison:

Proposal v4:

Easing icons only:

So that's it. I wait your comments on these changes. Thanks @Joshua Leung (aligorith) and Thomas @Thomas Beck (plasmasolutions) for giving your opinion and advising.

Hi @Paulo José Oliveira Amaro (pauloup),

Nice work with the comparison. Indeed, with white, the contrast isn't that great.

v4 looks good to me :)

@Thomas Beck (plasmasolutions): Unless you've anything to add, I think these are ready to be committed

@Joshua Leung (aligorith) Yeah, they are lovely and ready from my side of view :) Would you like to commit them? I'm away from my machine now...
@Paulo José Oliveira Amaro (pauloup) I like them very much and thank you very much for taking the time to make and alter them!

Greetings & Thanks, Thomas

I'm not familiar with the easing itself, but the Bounce and Back icons feel odd - I expect them to be mirrored (left and right flipped for Bounce, top and bottom for Back).

If I picture a bouncing ball, it should first bounce a lot a the beginning, then less each time until all kinetic energy is used up.
If I picture a Back easing for this: [start] ____/‾‾‾‾‾‾ [end], I expect it to go from [start] at left bottom to right top, but exceed the target on Y-axis, then go down a bit to [end].

But it depends on what is actually done in the easing.

If I compare the reference icons with the proposed v4 icons, the ref icons seem to stand out more. They got some kind of highlight or embossing (chisel effect?). They neither look flat, nor completely round, more like a flat relief.

Other than that, they are awesome!

Hi @codemanx, I tried to represent the In mode in these icons, so they are Bounce In and Back in. I believe they match to the reference in: http://easings.net/
Maybe Bounce should use the Out mode. I thinks it's used more often, I'm not sure if it applies to Back also. Therefore it would make it inconsistent to the other modes.

About the ref icons seem to stand out more, I believe it's because they are sharper, plus the highlight effect. So I tried to get as closer as I can in this new proposal (v5).

Now the lines are thinner and sharper, have highlight on top and the border is stronger. The changes are subtle. Adding effects can affect the color perception.
A comparison between the last version v4 and the new v5:

Easing icons only:

It's up to you guys. Any suggestion?

@Paulo José Oliveira Amaro (pauloup) From my side we're good to go - I think that the latter version is marginally better... but all in all they match the functions very well (and I think the bounce icon is well suited for it).
@codemanx This depends entirely if you choose the (in) or the (out) version of the bounce or back effect. The icons represent the actual working of those effects quite well.

I vote for "put them in"..

Makes sense, as the input mode is shown, it's absolutely correct!

The numbers in v5 are better readable on white background, that's a good improvement. Put them in!

I'm very happy to be part of Blender development. This is being my first contribution to Blender actually! :-D And even more because it's for a feature that I like so much! Thank you @Joshua Leung (aligorith), @codemanx and Thomas @Thomas Beck (plasmasolutions) for the suggestions and comments on my icons. I'm sure I could not be so satisfied with the result as I am if you didn't have guided me so well as you did. Thank you so much! :-D

I think I speak for @Joshua Leung (aligorith) too when I say that it's encouraging to hear that our work is helping users and is used to create any kind of art :) And I'm very happy that we are in the lucky position of having users that are able to produce art that in return is added to blender - for me this is true open source collaboration and makes me feel great!
Btw: This is how the icons were commented by Stephen Thomas on twitter, so I think you did your job well ;) " The new icons for the interpolation modes in #b3d are so tasty I want to lick them. Yum :) pic.twitter.com/BrRNZK3iiF "

I'm hoping to see more icons in other tasks in the future ;)

Thank you! Very nice to know it and thank you for let me know it!

Yeah, I'm totally available, if you know something I could help, just let me know. I actually tried to build blender from the source, to start to learn how to help as a coder, but I was not successful to solve its dependences. If someone could help me, I would be very grateful. My email is pauloup @ gmail.com and some guidance or even tips are very welcome.

@Paulo José Oliveira Amaro (pauloup) Very nice icons.
I really like the quality of the blender icon set. Sometimes icons are reused more than one place though so its nice of you to give a hand. I feel your style give some extra color to the original style. Looks tasty :)

A small comment. Is it me or should the sine icon be more symmetric so to communicate the sine-wave effect?
Maybe I have misunderstood the sine easing...?

@Ejnar Brendsdal (ejnaren), it seems your doubt is similar to @codemanx's one. Maybe you're thinking about the "In Out" mode of sine. (See easeInOutSine here). The easing icons represent the "In" mode only.

@Paulo José Oliveira Amaro (pauloup) Ahh I see. It was me who misunderstood the idea :) So I see it makes sense. And apparently it does communicate the "in" mode well. Otherwise I would not have recognized it as odd.