I've struggle an entire day but I finally got it.
I've submitted them my 2 most used free asset (ground & metal) on Unity.
Let's hope it will hit their attention (not too hard ^^ )
The Unreal submission is a bit complicated:
1- you submit your suggestion with images;
2- they decide if it's interesting enough (private voting);
3- if yes, they post it on a dedicated site for public voting;
4- then if public is okay (enough votes) they contact you for further instructions; =S
Man, they're giving themselves too much work for that.
Unity submission workflow is less tricky,
You send everything, if it's not good enough or already present in big amount & variety, they just don't validate your submission.
I guess they will figure it out soon enough.