Well,
a project like MOBILE SUIT GUNDAM THUNDERBOLT commercialised on UHD BD is actually produced at 1440×810 for the conformed Digital Intermediate before the video upscaling done in authoring (there are other benefits for the use of the format, of course), but it is worth noting how, for example, the rendering of scenes integrating mostly animation of CGI assets are a more achievable prospect to be produced with higher base resolution targets in mind... than what has to be done with
the line-art which is the main limiting factor on itself. All-time proponents of full digital animation like '.GIF generation' (or 'web-gen') godfather
ryo-timo have been talking about the compromises derived from the (still necessary practically on every project) step of scanning paper sheets, of which it is very difficult to extract information easily usable for impending new standards. Using the 'AJA Fr01' A4 layout paper standard from
The Association of Japanese Animations (
http://aja.gr.jp/jigyou/chousa/aja_layout), that is distributed in files featuring graphs rated at 72 DPI and devised originally for a 720×405 base resolution in mind, for FHD to be resolved while keeping comparably fine detail intact data at 192 DPI must be obtained. To more comfortably work on such prospective ends paper sheets beyond A2 size should be used! However, and as
Theonik mentions for instances like FX, composing each frame depends of several elements that could have been rendered variably, so the final result isn't entirely dictated by line-art alone;
RURIN's famous spectrum analyses pointed how some scenes featuring mostly animation or rigging with 3DCGI models overall could be rendered at differing higher resolutions, on shows like Evangelion: 3.0.
There is also the matter of how it is actually possible to extract additional detail at 6K or beyond from 'Telecine' scans on old film stock at non-narrow gauge, depending on the kind of capture equipment used. What was crammed visually into the cels of some productions from
Studio Ghibli (as an usual example) and photographically transferred into 35mm film can be now superiorly replicated in newer digital master conversions,
that even could have extend dynamic range for reproduction to take advantage of more recent standards for AV consumer electronics products.