Ongoing:

• More Tests -- Any tests added on their own. (painless: add trap tests #18538) (Tests: Make painless score tests unit tests #18560)
• Any Bug Fixes -- Any and all bug fixes. (Fix Casting Bug #18871) (elastic/x-plugins/issues/2587)
• General Usability - Improve syntax for usability. (Add synthetic length property as alias to Lists, so they can be used like arrays #18241) (Remove input, support params instead #18287) (elastic/x-plugins/pull/2225)
• Better Dev - Improve ease of development. (Add debugging tool to tests #18288) (Use isAssignableFrom instead of relying on ClassCastException #18350) (Definition cleanup #18463) (Refactor painless variables handling #18951) (Change Painless Tree Structure for Variable/Method Chains #19459)
• Def Performance - Improve performance for dynamic types.
  (painless: optimize/simplify dynamic field and method access #18151) (Switch painless dynamic calls to invokedynamic, remove perf hack/cheat #18201) (Array load/store and length with invokedynamic #18232) (Use better typing for dynamic method calls #18234) (Remove boxing when loading and storing values in "def" fields/arrays, remove boxing onsimple method calls of "def" methods #18359) (Fix Bug in Painless Assignment #18379) (Made def variable casting consistent with invokedynamic rules #18425) (Remove casts and boxing for dynamic math #18849) (improve unary operators and cleanup tests #18867) (fix bugs in operators and more improvements for the dynamic case #18899) (Fix explicit casts and improve tests. #18958)
• General Performance - Improve overall performance. (Implement needsScore() correctly. #18247) (Painless doc access #18262) (Add 'ctx' keyword to painless. #18264) (_value support in painess? #18284)
• Grammar Changes -- Any and all grammar changes. (Remove Grammar Ambiguities #18531) (Add } as a delimiter.  #18827) (Move semicolon hack into lexer #18931) (cutover some docs to painless #19092)
• Better Error Messages - Make error messages bubble to the top level. Make better error messages for syntax errors caught during the ANTLR lexing/parsing phases. (Make Line Number Available in Painless #18298) (Improve exception stacktraces #18319) (replace ScriptException with a better one #18600) (Improve painless compile-time exceptions #18711)
• Anything Else - Any other changes not falling into a category. (Catch OutOfMemory and StackOverflow errors in Painless #19936)

Documentation:

• Split Pages: Painless has a large feature set, so it needs to be split into multiple pages.
• New Examples: Painless documentation needs new examples.
• Updated Examples: Update the script examples used by other pieces of Elasticsearch to use Painless instead. (elastic/x-plugins/pull/2327) (elastic/x-plugins/pull/2449) (elastic/x-plugins/pull/2571)

Tasks:

• Date API - Add a Date API. (add java.time packages to painless whitelist #18621) (improve date api for expressions/painless fields #18658)
• Internal AST -- Decouple ANTLR from Painless to allow for other possible uses of the compilation unit. (Decouple ANTLR AST from Painless #18286)
• Generics - Remove all generics for simplicity.
• Overload - Allow for arity overloaded methods/constructors. (Add method overloading based on arity #18385)
• Regular Expressions - Add some form of regular expressions. (Add support for /regex/ #18842) (Add support for the find operator (=~) and the match operator (==~) #18858) (Add replaceAll and replaceFirst #19070)
• Strings - Allow single-quoted strings to be used. (Painless: Single-Quoted Strings #18150)
• Foreach - Allow the foreach shortcut in for loops. (Static For Each #18757)
• Properties - Expose properties for the different field types including geopoint support (Add fielddata accessors (.value/.values/.distance()/etc) #18169)
• .value - Add .value for field access. (Add fielddata accessors (.value/.values/.distance()/etc) #18169)
• Improved API -- Any updates to the whitelist. (Whitelist expansion #18372) (improve painless whitelist coverage of java api #18533)
• Colon Syntax -- Add method pointers using colon syntax.
• Lambda Functions -- Anything lambda related. (Add Function Reference Stub to Painless #18578) (Method reference support #18748) (Add capturing method references #18818) (Add Lambda Stub Node #18824) (Method references to user functions #18828) (non-capturing lambda support #18911) (improve lambda syntax (allow single expression) #18924) (Add lambda captures #18954) (Infer lambda arguments/return type #18983) (Add augmentation #19003)
• Custom Functions -- Add user defined functions. (Add Functions to Painless #18810)
• Type Initializers -- Add initializers for arrays, lists, and maps. (Painless Initializers #19012)
• Instanceof Operator -- Add an instanceof operator. (Bring painless docs closer to reality #19065)