I guess you've subconsciously mixed up survival and suicidal like I always do...
In my opinion, there need to be a few more difficulties in the mix. One even harder than HOE for when you basically have a team of hardened veterans who want to try-hard and still be particularly challenged. Maybe even an adaptive difficulty which gets arbitrarily harder and harder every time you've beaten it so that there's extra replay-ability for the squads who can beat HOE every time without fail. I'm sure many of those teams would then go on to "unlock" those bad-*** difficulties and see how far they can push it and feel proud to say that their squads have hit whatever score on the Richter scale of difficulty...
The other difficulty I feel should be added is one between hard and suicidal as I feel the gap may be too big for players who feel they're beginning to master hard and want a new challenge. I reckon that's why so many people are getting kicked from suicidal servers and are confused about why the hell they even died at times...
Yeah my fault I do get then mixed up. I also agree there should be another difficulty, one that they can make as hard as they want where fps and scrakes spawned raged and husks just run after you to suicide lol