There are road pull 11 speed flat bar shifters, so you can use your 5800 mech but they're quite pricey, but I'd probably pick up whatever MTB 11 speed stuff you can find, deore is fine but SLX is nicer, XT, especially the shifter is best but it all works well, deore shifter is a bit plasticy, can mix and match the lot.
There are road pull 11 speed flat bar shifters, so you can use your 5800 mech but they're quite pricey, but I'd probably pick up whatever MTB 11 speed stuff you can find, deore is fine but SLX is nicer, XT, especially the shifter is best but it all works well, deore shifter is a bit plasticy, can mix and match the lot.