In a way that's kind of what SuperNET is doing.
Also, there is the Jay framework:
http://jnxt.org/jay/Thanks, I didn't know Jay. If it is safe/stable enough for merchants, it can be a good solution for them.
SuperNET is a bit off my radar and it surprises me that you mention it, as I thought it was a pretty heavy solution. Must take a read about it.
Anyway, I would really like to see a minimalistic Nxt command line client with only the core features

I meant more like SuperNET is the advanced client with all the plugins, not as a lightweight client. Although Jay is part of SuperNET, and there are two different presentations of the SuperNET: a standard easy to use mode, and an advanced mode with access to everything. I wouldn't call the SuperNET client itself lightweight though as it is built on top of NXT NRS.