Here's what I came up with for now:
- Support for at least PCX, BMP, GIF, PNG input files (as it's a pain to convert your old BMP to PCX first just because the tool you need to use in this case doesn't support the former)
- Support for optionally (!) optimizing the source data for the SNES tile format (i.e., by reusing redundant tiles)
- Support for outputting tile maps and palettes (possibly even in either binary or text format, with a user-selectable .DB-like prefix) along with the raw tile data
- Support for outputting 1bpp, 2bpp, 3bpp, 4bpp, 7bpp, 8bpp, and Mode 7 gfx data
- Full tile map configuration support, e.g. the user may add offsets to both tile and palette numbers and/or specify to clear or set any of the X/Y flip bits
- Support for outputting tile maps in either "interleaved" or "non-interleaved" format (i.e., for a screen of 32×32 tiles, you'd either get a single 2 KiB file with [$2118/9 16-bit data *1024] or two 1 KiB files containing [$2118 8-bit data *1024] and [$2119 8-bit data *1024], respectively)
- Support for outputting Mode 5/6-compatible tile maps (i.e. tile maps where odd tile numbers are skipped entirely since these modes force 16×8 tiles anyway) -- or, even better -- let the user decide whether they want their tile map to be based on 8×8, 16×8, or 16×16 tiles (which should cover all BG-based cases … sprites would be a different story, of course)
- Possibly even support for pictures using the full RGB color range, with optional dithering and what not …
Feel free to add your own suggestions to this thread. Maybe this will inspire someone to tackle a project like this in the future.