3200, even something like Delta 3200 which is really 1000 with low contrast so very pushable, would be interesting in LF sheets.
HIE and Type 55 definitely but both have special challenges. If HIE were easy to make Efke would be making it instead of IR film that as an effective filtered speed of 1.5-3, as Brian says.
There might be a market for a slow film in 4x5 (but Adox and Efke have them, no idea if they're any good as I haven't tried them) but TMX is so good, as long as Kodak is around there isn't much use in going slower. Delta 100, while not as fine grained as TMX, probably also beats old style 25-50 speed emulsions, probably with the exception of the lamented APX 25. Some would like that but it's just too slow for me.
Portra 400 and Ektar.
There just don't seem to me to be any real "holes" in B&W. Even if Kodak goes away I think pursuading Ilford to bring back Delta 400 (and maybe XP2 Super as well) in sheets would have more chance of success and be far easier than making something like TMY-2. Not that Delta 400 is a replacement for TMY-2 (before I get outraged comments telling me that.) I know it isn't, but it's finer grained than old style films of 400 speed.
There's a lot missing or potentially about to be missing in color, not so much in black and white which is well supplied. Make something like Astia. I just shot my first roll of it in 120, having used it some in 35mm, and it's such a lovely film, a real sham to lose it. OTOH it isn't going to be wet printable without internegs now that Ilfochrome is gone, so maybe bring back type R paper. I know I keep saying that, but I miss it.
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