Different instrument completely. Different tone colour, range, diverse capabilities in every way - I like French horns btw - can you tell!!!?
My thinking is they could never fit in the current model, there's have to be another part - a French Horn part!
Don't get me wrong. I like french horns too. I mean, I like Chorizo. I also like custard. That doesn't mean I want them both on the same plate though.....
The long and the short of it is that yes, the french horn is a very fine instrument, and fits into the various ensembles it is included in (orchestras, wind bands, military bands, brass choirs, brass quintets etc) very well. A large part of the reason it fits in so well is because these ensemble include the french horn rather than the saxhorn that covers roughly the same pitch.
I think we're possibly missing a point here by pitching the french horn against the tenor and baritone saxhorn, when in reality the range and dexterity demanded by the player are very similar to the role in which a euphonium sits.....
....except we've already got euphoniums....
A few posters have already outlined why replacing existing EB and Bb saxhorns with F horns doesn't make sense, and the more I look at it, for everything that a french horn could bring to a brass band we already have and instument within a brass band that fills that gap. And I'm not sure an 'Add horns and stir' method would work for classics of the repertoire such as resurgam or epic symphony......
The cons against inclusion outweigh the pros in my opinion.
As for rolling out the Bb D, Eb, A and C trumpets, just no.... really... no. Cornets are perfectly good at the job they do and I've never known a trumpet work in brass band context. It has the same effect as including a front-action tuba in a bass section by completely destroying the homogeneous sound a section is designed to create.
I've nothing at all against anyone out there experimenting with instrumentation and including whatever instruments you like. They all have their merits and detriments.....
...Just don't expect me to call it a brass band when you do, because it isn't.