Requirements

• Split images MUST NOT be supported anymore. Add check that split images are not allowed anymore (syntax check). Alternative would be to use the SVG output format with SVG out plugin. 
• Add global admin configuration for default image format (PNG or SVG) and function to flush the cache.
• HTML image maps MUST be supported. 
• No special requirements for cluster mode (SUPPORTED but no special implementation)
• Provide some performance logging when log level is set to DEBUG.
• Only cache static macros e.g. the {plantuml}version{plantuml} output or {spacegraph}, {linkgraph}, {database-structure}, {database-info} MUST NOT be cached since the output may change on every request. (isCachingAllowed)
• Global cache configuration MUST be supported as default for all relevant macros (isCachingEnabled).

Algorithm 

var image;

if (isCachingAllowed() && isCachingEnabled()) {
   var cache = cacheProvider.getCache()
   var key = hash(macro.source)
   if (!cache.contains(key)) {
      var renderedImage = render()
      cache.put(key, renderedImage)
   }
   image = cache.get(key)
} else {
   image = render()
}

Cache Storage

• In memory Confluence Cache (default/preferred) supports cluster (MUST)
• File System (/path/to/cache/directory). Is the max files per directory limit a problem? No cleanup of outdated files. Cache directory inside $confluence.home or configurable (admin section of the plugin) (MUST)
• External server (like plantuml.com). Either self hosted or external plantuml-server? no native graphviz installation required but setup is more complex! (OPTIONAL) 
• Page attachment. exportName parameter already exists but not the corresponding source hash. store source hash as page property? History of the attachments is not always wanted. (OPTIONAL) 

How to configure/specify the cache as macro parameter?

{plantuml:cache=<empty>|none|attachment|fs|server|...)

<empty>  → could be the default cache storage e.g. in memory cache