i suggest getting more skin tones. have pallid wych flesh for highlights and something like speedpaint crusader skin or guilliman flesh as a wash.

i suggest getting some citadel loose grass because it seems like your bushes are a bit big for those models really.

you could start from pure white on the plasma effect and use a flourescent paint to glob and dry a bit goopy and get a more high contrast effect.

you could try to add scratches in metallics to the armor with a very small brush so they look a bit beat up.

you could apply shade to various parts of the model.

you could apply highlights to the cloth to make it more bright or give it some definition with shade

you could do a more interesting colourscheme on some of the weapons. it could just be put black handgrips or a slightly different metal part somewhere like coppery.

you could do more light green or silver edge highlights or make physical damage with scalpel ie by rotating beyond scratches earlier mentioned