I’m not a great user of presets, but wouldn’t want to see the thumbnails any smaller than they are now. My reason is that if they became too small you will not be able to determine if the preset is something you might want to apply, in which case the thumbnail becomes redundant.
I don’t have any particular requirement for additional presets.
I’m happy with current situation. If something change, it should be at user specifications for number and size of vignettes.
Perhaps some night rendering and portraits.
Q1: Seems good as is but am open to change 
Using 3240x2160 15”. Please note I do not use the thumbnail preview for my use of previews is generic editing of RAW into TIFF (lens, cam body, ISO, Fstop combinations), not so much for creative sake this step would be entered manually. Where i would love some thumbnails however is with viewing ICC profiles and alike (colour rendering). At the moment I need to manually go through the list and memorise what my faves were then go back and find them. Seeing them all via thumbnail previews would be a game changer
Q2: As mentioned above I love and use custom profiles. Is there a way to get custom profiles above the official profiles?
Kind Regards
Thanks for your feedback! 
Assuming that you still think of presets: You can create a folder that holds your personal presets. Give the folder a name that makes DPL put the folder on top of the others. If the name starts with a “0”, the folder will be on top of everything else, if the name starts with a character (a-z), the folder will appear below the other folders.
Rename your presets accordingly to make them appear just below the folders and above DxO’s presets if you don’t want an additional folder…
Q1:
Personally I always apply a preset to properly judge it.
I only use my own presets, especially since noise reduction settings etc. are always included and cannot be ignored when applying a preset. So I moved the ‘general purpose’ presets to a folder so my own presets would be at top. But they always re-appear and as a result I always have to scroll down for my own presets. So YES, I’d love smaller preset thumbnails 
Q2:
Not interested
I’d really love to be able to load only a part of a preset. When there’s perspective correction and even local adjustments, they are all removed when loading a preset.
Thanks for your feedback!
