unit files barely have any nesting, so the INI-like format is already 90% of the way towards TOML, no?

I suppose some things that can be multiply specified would be arrays, and maybe there'd be a stronger argument for combining say timer & service in the same file (or that it could be optional) given some structure.

Personally though I'm not really a fan of TOML when the data is highly structured - prefer YAML even a lot of the time for some visual indication. (If I were writing a parser, I have zero doubt I'd rather parse TOML!)