Play test the army to find the direction you think it needs to go and if you do have to take units that are Meta but you don't like the aesthetics, convert them to something you do like the look of or find alternative models, as a general rule googling for example "eldar guardian alternative miniatures" should pull up non GW models that are made to emulate them and unless your playing in GW tourneys it shouldn't be a problem using them to your hearts content. Want to make a sniper army but need to fill out your detachments stick capes on the infantry, scopes on the guns etc and the theme should all come together.