OpenFX (ofx) is an open standard for 2D visual plugins and a lot of software supports it.
Many plugins is written for it and many is converted from other formats.
The open source compositor Ramen is writing many of its visual effect nodes as ofx-plugins.
In short, there are many high-end plugins for ofx and new onescreated every day.

Could Kdenlive use it?

http://openfx.sourceforge.net/

Someone needs to write a bridge in MLT for it like Marco Gittler did for frei0r, but I am quite busy with other things related to MLT. Frei0r is popular, easy to download and contribute to, active, packaged for most distros, and its plugins fully open source. Can the same be said for OpenFX? I looked, and it is not at all obvious where to even get plugins. In fact, a google search on "openfx" yields this page on the first page of hits! Most of the other hits refer to an 3D modeller.

OK, I see quite a few expensive proprietary ones at http://toolfarm.stores.yahoo.net/ofx.html

Unless someone contributes one, I am only open to a well-paid contract to develop an OFX module for MLT.