PlantUML to my knowledge as on date supports (Sequence, use case, class, activity, component, state & object diagrams). However i find it difficult to complete switch and use only plantuml for all my design and architectural activites. For that i would support for creation
Context diagrams (Block diagrams)
Architecture Diagrams (Block diagrams with North bound and South bound gateways)
System composition and System decomposition digram
Sub systems interaction diagrams
Sometimes process flow diagrams...
Stacked block diagrams and much more..