I'll try to give my opinion to the mentioned things in op.

headshots: Would be nice if they were contributable, but the biggest problem I see is: We contribute profiles for a Single DVD, BR, ...  so I think it is hard to manage how these are treated. With an actor/crew database, it would look different. What remains are probs in ruling and copryright problems.

images for galleries: Nearly the same as explained at headshots.

rerelease scans: Would be also nice. Something like a list to choose which variant of cover we want in our local database. But this would lead into the next: Variants of profiles. It's not always just the cover that differs, sometimes a rerelease misses an bonus disc or normal cover instead of Slip cover, ... If we could manage to make different variants of to one EAN/UPC we could do also such things. Btw, then would would also be able to manage different movies with the same UPC.

genres: Imho one of those things that are mostly influenced by personnel taste, so hard to contribute custom Genres.

dividers: In cast if once understood not too hard too use. Could also live without them, but also not bad to have them.

bold and italics in overviews: Until this remains the only possibilities we have to format Overviews it is Ok. I see this more as help for reading, than to reflect the back cover. So not the worst feature. But never add different font heights, colours, ... Would see no help in this.

boxsets: Why not? It is good to have included movies separate.

totally useless crew roles for the "smallest" ones: Hmm, I simple ignore those roles I'm not interested in, so I'm not perfect to give an opinion on this. But if there's some more or less in a profile doesn't bother me this much. Never did much thinking on this. Always hoped the roles we allow are enough and not too much.