Almost all of the world uses "foreign" systems to write their languages.

Well, most of Europe anyway, Europeans use Semitic letters. It's a whole other story with Middle-Eastern and Asian countries, they use their own. Then there are the writing systems of the Americas, Africa, and Australia.

In the past, Slavs used simple tallies to count, and simple sketches of animals, plants and other things to mark good hunting and fishing areas as well as territory and these were maybe even used to represent abstract ideas (ideograms).
This system would have eventually evolved into a system of more complicated and complex Ideograms, Logograms and Pictograms, and then perhaps into an alphabet.
Slavic linguist should get together, and use these symbols (there are many) to make a new writing system for all Slavs. It would start of as a semanto-phonetic writing system, and then if necessary evolve into something else.
[img width=428 height=700]http://s13.postimg.org/zfvsakpjr/hrn_iarske_zna_ky.jpg” />
[img width=700 height=324]http://s13.postimage.org/ljlcr3pfr/ris3.jpg” />
[img width=384 height=700]http://s14.postimg.org/q4vz03o75/ris6.jpg” />