So I think you've kind of hit on part of what I (OP) was asking. Maybe I should have framed the question of: If we are looking to fill gaps which exist in the heritage railway story (as arguably the LMS Patriot would, as 82045 does for example as one of the missing standard classes, and the Bluebell's 84xxx), which missing classes would represent a sensible investment for the current heritage scene? Either because they'd suit a particular line's theme or story (Bluebell, Southern) or because they offer something that would work for various railways (the LNER G5 which could theoretically be at home on many heritage railways). So I'd argue that whilst the LNER W1, or Big Bertha, are significant and unique in British railway heritage, but I'd also argue that building a new replica of those would not be a sensible investment - even the A1 Trust seems to have recognised that it needs to supplement its two high-powered express engines with a more versatile V4. The assumption was made that a viable funding plan and engineering team could be implemented!