I found some ideas that have already been submitted as Feature Request issues on github. Many of these are too small to be a whole project, but two or three related ones could be grouped together.
comm/timing diagnostics:
Support more flexible CommDiagnostics I/O #16958 bradcray
Implement comm diagnostics for network atomics #10124 ronawho
Combine CommDiagnostics and VisualDebug #5919 ben-albrecht
Improve maintainability of "comm count" tests #6137 ronawho
(at least the idea in the description, maybe not the later commentary.)
Feature request : timing annotations #15878 npadmana
Implement monotonic timers #7536 ronawho
proposed modules / bindings:
Provide Stochastic Gradient Descent in some ML package #6847 buddha314
Develop Probability distributions a' la R, SciPy, Spark, etc #6852 buddha314
Chapel bindings for GSL - GNU's Probability and Statistics Library #12743 LouisJenkinsCS
(At least begun by npadmana)
Support Pandas-like DataFrame #7187 buddha314
(Not sure what's left after #9042)
Improving Crypto module #7948 lprimeroo
Implement the Porter Stemmer in Chapel #8336 buddha314
Create a Geometry library #9069 buddha314
Improve MPI Module #5722 ben-albrecht
Improve LAPACK module #5687 ben-albrecht
Develop Logging Suite for Chapel #7556 buddha314
Feature Request: Logger framework that is parallel-safe, scalable, and distributed #10804 LouisJenkinsCS
Build REST API layer for Chapel #7584 buddha314
(Begun by marcosleison )
Support argparse-like functionality #8302 buddha314
Env module #5939 ben-albrecht
SQL or ODBC support module #6800 buddha314
Implement the Bolt Protocol for DB communication #7069 buddha314
(Existing CDO project proposal may cover these?)
Create a mutex/lock library #9881 ronawho
iterators:
Feature Request: Utility 'unique' function that removes duplicate elements from an iterable #10665 LouisJenkinsCS
Add dense followers to sparse arrays/domains #13452 bradcray
Add DefaultRectangular iterators that iterate in CMO #11377 daviditen
development integration:
Support Jupyter Kernel for Chapel #7414 buddha314
Create Language Server #7295 marcoscleison
Python interop: Expose Chapel records as Python objects #14420 ben-albrecht
language/compiler:
Support 'in' as an infix operator for membership test #5034 ben-albrecht
Request: select-when with compound when clauses #15048 BryantLam
Add support for serial leader iterators #13665 bradcray
short string optimization #11221 mppf