What would be the next step up from the Panny 100-300mm on my OM-D??

I currently have the above combination- and am considering what I could do to improve from there? I am unfamilar with the differences between adapting a 4/3 lens to the M43 vs. adapting SLR/DSLR glass to M43.... is 4/3 glass a better match to the OMD than SLR/DSLR glass? If I wanted a better lens than the Panny 100-300- any recommendations? Looking for sharper images at max range within a reasonable price range.

Thanks for any advice-

Wes