SDL applications will only offer input method support through fcitx (I believe it's the version 4 series, but I'm not sure) - See FL-9474. So it's not just a matter of preference between ibus and fcitx, there are actually cases in which one or the other is required.

Gentoo support for fcitx is not very good. fcitx5 only exists in unofficial overlays, like zugaina and gentoo-zh. There is also a discussion on whether fcitx4 and fcixt5 should be in the same catpkg with different slots or if they should be independent packages, as every other distribution I know of has them.

To add to the complexity there are the different GUI toolkits (gtk2, gtk3, qt5, sdl) and display systems (X and Wayland). There is also discussion on whether the support for some of them should be added by different packages or by use flags in one single ebuild.

As I starting point, I would consider this issue to be resolved if an ebuild with support for fcitx4 for gtk4 and qt5 on X can be added to Funtoo, even if it's just forking the one from Gentoo. But it's probably best to consider using the ones from the gentoo-zh overlay, as opposed to the official ones from Gentoo's portage tree.

However to fulfill the Multilingual Project full ideal, a lot more study and discussion will be necessary and just bringing ebuilds from Gentoo won't do it. These will need to be Funtoo specific ebuilds.