I approve of the allowance of a spin-off tab simply because I think it would good for original creators to organize their work if their stories take place in the same universe.
For the fan fiction tab, thou I love it a lot but I would say NO. Not on the popular tab nor trending nor a genre. I still think it should be allowed but not promoted.
My reasons are as follows:
1) Generally, people would follow fanfics more often than original, and as seen with smakejeeve the site was and still is DOMINATED by pokemon and Steven universe fan comic. Frontpage poplar and just fan comic. Tapas and Line are the few places where original comics dominate and encourage original creators in general. While sites that allow fan comics to be front-page discourage this since if your fan comic gets more attention than your original many would be discouraged to continue the original
2) It might get the site in some hot water to actively promote it with a genre rather than just casually have them but not give them much attention
3) Fanfics have so many avenues already that would allow and cater to them. AO, Fancfic.net, Soo many fan sites for specific fandoms, tumbler, WattPad. While catering for originals are so rare and the ones that claim to be so like Watt Pad and Smack Jeeves are dominated by fanfics. Thus overall I think we should have at least one popular place like here to not allow it and cater for originals.
I love fanfics and I may one day try writing one again someday but I would post it on the many sites that cater to it.