You could do a xenos scout army; Kroot, Pathfinders, Darkstrider, Tetras, Illic Nightspear, Pathfinders/Rangers....
I really like pathfinders, but i've bought a load of Tau before and lost interest before i even got started. It's a while before i start this project so i'll give it a good bit of consideration.
I have a SM Scout army, I have them mixed with IG & Sentinels (use the IG as allies) painted up as scouts with a mixture of scout and IG bits n bobs, but in my mind the IG's are scouts
This sentinel idea sounds very interesting, i shall have to look into this!