Gotta get this out of my system before I do my routine...
After reading all the comments above, I think we can go that route of not using "mode" on special outfits, especially moving forward. This is the system already applied to those associated with brands and cities with IRL events. But removing "mode" from already existing tags can be a pain since the BUR system does not allow wildcards.
As for putting under the different outfits for the remodels under the base tag as Evasion suggested, that would be best tackled on a case-to-case basis especially if there is the possibility of a base form "cosplaying" as her "remodeled" self, especially if the outfit is only available to a remodel (one case in point is post #5030034). I already brought this up back in forum #292959. Even remodels get their own cosplay tag now.
We should also be wary though that some outfits can "bleed" outside their designation in Kancolle Wiki, especially in fan art, hence them being named differently here. Harusame's maid outfit is officially her Valentine's outfit, but post #3851074 is set during Hanami. Also, Isonami's photographer outfit is officially her autumn outfit, but post #3846691 and post #5238947 has her in spring. Finally, in Kancolle Wiki, for many characters, summer CG=swimsuit CG, but not all; even Yukikaze has separate official swimsuit and summer CGs. And not all new year's CGs feature fur-trimmed haregi, e.g. Akebono and Akigumo.
Looking back, there are some tags that do need to be merged and/or aliased. In principle, outfits across forms warrant separate tags when they there are at least two elements added or removed to a base outfit, but when the only change is the addition of/change to only one element, only the base tag should be used. This is why there's the alias request above for Italia's swimsuit form back to Littorio's (the absence of the sarong), although depopulating the posts to the base form can suffice. Sagiri's swimsuit can be treated on the same way as well even though it is the other way around (Sagiri's Kai swimsuit CG is basically the base one without the halter top).
But as usual, I need help on applying new tags when found and making the dummy wikis. We are now doing this at a time several Kancolle artists and the general fandom have slowly moved on to other franchises almost completely, notably Ido, Negahami, and Hamu Kotarou, and the upload activity on the Kancolle front over the past year or so reflects that.