I think of Diala as the glue that holds the party together. She isn't as good at attacking as other dedicated melee heroes due to her not having a gap closer. However, she brings a lot of other skills to the table that helps with general action economy and utility. If you have to change, you could take a dedicated damage dealer instead, but you will lose a lot in the transition.
Shyla and Garkhann close gaps in very different ways. I like Shyla more than I like Garkhaan due to her movement and placing shenanigans, but they are on about equal terms in power level.
MHD-19 is a solid semi-support/utility, I find. He can heal party members and do the occasional focused attack. I would rather go with Murne as the full support, I think.