I think that would be a good idea. I think there was one at SourceForge, but it was rather obscure, and nobody responded to requests (maybe an exaggeration, but that was how it seemed).
IMHO a suggestions thread only works if a reasonable number of people respond to it - for example, a suggestion may get no support, or it may get approval from others, or maybe improved.
Also many requests may be satisfied simply by pointing to a suitable plugin.
I am not talking about proposed NPP source code changes that some people offer - this probably requires a more formal mechanism.
David