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System Verilog and/or VHDL support for RTLBrowse. Currently it's only plain-vanilla verilog from the 1995 spec.

More “interesting” integrations with other tools. The new Tcl interface adds a bunch of possibilities. I know one guy is using it to allow remote control from emacs through a bridge server.

Improved analog support. This would also mean being able to read various spice rawfiles, etc.

An open source equivalent to FSDB in terms of speed, file size, and capabilities of the different types of data it can store. This wouldn't necessarily be a gtkwave-only project given that I'd expect a reader/writer API that for example, icarus/ghdl and gtkwave can interface to. This is the most challenging of the bunch and would require not only coding, but lots of experimentation + throwing away code + restarting over when it's realized a better way can be used to do something.