This is my first contribution.
The original photo is courtesy of Issabella:



My outcome is the following:



This result has been obtained by:
1-calling the garagecoder's filter gcd_sharpen_tones with the parameters: 2 and 100
2-calling the filter orientation to create a new layer, then normalizing the result and blending it (overlay, 0.6 opacity) with the output of GC filter
3-applying a gamma modification taking into account the average value and the goal value (in my case 100), then normalizing again
I stored the (small) series of steps in a personal filter called sharpenenhance , which I will send to GC, once the winter will start and he will switch from music to Gimp-chat again, to let him tune it and -if considered acceptable- to release as a modified version of his filter.

My second contribution:

original (one of my summer photos):



enhanced outcome:



obtained by:
- applying Iain's filter Tone Presets Warm Vintage on a new layer
- modifying layer mode to Overlay 100%
- duplicating this layer
- changing layer mode to Multiply

Great improvement in both photos Dinasset, I keep the first one, I love it!

Welcome back, Issabella!
I missed you (as well as many other here around, I'm sure)
Thanks for your appreciation.

by walking over G'MIC filters, I noticed that some photos can be enhanced by using filters more specifically devoted to "transform" them, better than to enhance them.
Here an example:
original


enhanced by using wild-cartoonizer (after perspective adjustment)


I like how the details have been improved

using another filter on the same base image, local orientation, but -while in the previous example I just took the output of cartoonizer as it was- in this case the output of the filter has been overlayed on the base

Good investigation, Dinasset. I take note about your founds. Notable improvement.

gracias, mi amiga de Espana!
sometimes one should abandon the well known roads and try other paths...
it happens you find something interesting...