Systema is a Russian martial art practised by army special forces

The text "Systema" and "Slovenija" should be included in the logo
red 5 pointed star symbol is OK to include, but not necessary

I leave the choices to you, to be creative. 

For further, inspiration see google images for keywords: Slavic warrior, barbarian warrior

Also a good idea is to aim for something about special forces. Random ideas which you may find useful:

https://res.cloudinary.com/teepublic/image/private/s--x46CTUAa--/t_Preview/b_rgb:191919,c_limit,f_jpg,h_600,q_90,w_600/v1544251848/production/designs/3679625_0.jpg

https://yt3.ggpht.com/a/AGF-l7_Vh6V2vs_cYWFhuzyCcIX-2Cz-fE_5CMZ6Sw=s900-mo-c-c0xffffffff-rj-k-no

https://emblemsbf.com/img/6914.jpg

https://dumielauxepices.net/sites/default/files/dagger-clipart-special-force-498962-3063836.png

https://banner2.kisspng.com/20180406/qkq/kisspng-special-forces-military-united-states-special-oper-forcess-5ac78a89dcdc95.5493730515230265699047.jpg

https://www.pngfind.com/pngs/m/436-4369971_supporting-special-forces-royal-air-force-and.png

The logo will be worn on dark shirts so maybe use black background 

Systema Slovenija