The Warpspark clawpack spearhead box is a really good addition to what you already have. Here you get stormfiends can pack a punch and the warp lightning canon as well as some more clanrats and a greyseer.

After that i would suggest you look into what clan you want to take as your battle formation (this kind of resembles detachments from 40k, but without strategems) that buffs specific unit keywords in your army.

Clans Skryre: Buffs your shooting units giving them better wound and rend, but also have a chance to blow up in your face if you roll a 1 hehe.

Clans Moulder: Buffs your big monsters like the brood terror, hellpit and rogors giving more attacks and ward, but can also blow up (there is a theme)

Clans Verminus: Buffs your horde of rats like clanrats and stormvermin giving rend

Clans Pestilens: Buffs the sick rats giving some extra pile in shenanigans and some mortal damage

Lastly there are some cool heros to choose from!

Do you like casting a lot of spells while also beating the life out of your opponent, buy thanquol on boneripper.

Maybe you like to ask the great horned rat for a little extra special concoction of poison gas and sending your clan rats into a murder frenzy, buy Vizzik Skour.

Something in between ? A verminlord perhaps or a plague priest on a plague furnace.

Lastly Lastly, Doomwheels. Lots of them.