Keep it simple:
There have been plenty of suggestions for different classes and weapons, or even some ridiculous game modes. However, I think that some things should stay the same. First and foremost, there should be no more than one class (that has different abilities) for any team. The main game mechanic relies on this simple setup, even though it can lead to a lot of zany situations. This also means that the game can be easily accessible, so you can jump right in thinking, so this is black, and this is white, and I have to meet the goal so I can win.
As far as different weapons go, at most there should be one other weapon, one that you'd have to swap your standard books for, and would be somewhere around the map. Something supportive, like a blowback sort of tool, or maybe even a proximity mine so you can beat the shrubs at their own game. The key is to add to the variety while having the player make a choice and also have to move around to get to it. Respawn timers for weapons would be nifty, too.
However, that doesn't mean there couldn't be different types of shrubs or even survivors. There could be different, randomly selected models for each team that would be mainly for cosmetic purposes, although different shrubs would add to the challenge and paranoia factor.
Give us a goal:
One of the fundamental things that TFC and TF2 got right was the decision to remove the deathmatch entirely (except Arena mode, which still has a twist). For a game focused on teams or even something with an odd premise like Suicide Survival, Deathmatch gets repetitive soon, makes it easy to predict player strategies, and in the end, doesn't leave most players feel like they've contributed anything. I've also noticed through playing that most games turn into a camp-fest, since the DM game variant lets players get away with that very easily.
That's why I'd suggest a mode where there are multiple points scattered around the map for the suiciders to capture by standing on them for a set amount of time. The survivors have to strategize and split-up, and are also constantly on the move. On the suicider's side, they have to move quickly to eliminate the opposition before they can start capturing the point.
Throw in some more balance:
Right now, a game will either result in one team winning or the other, because of various advantages and disadvantages that throw off the balance of the game. Survivors can easily hold their own due to the seemingly limitless supply of ammo crates around the maps. So, we should remove these and make it so one would have to go over and walk over their books to pick it up and re-use it. Toss in a more restricting ammo limit and you now have created a more desperate and panicked situation on the survivor's side. But there's still the element of being able to very quickly dispatch your foes if you are a twitch-gamer, since books can one-hit-kill shrubs. So, we make it so one has to hold the primary fire in order to charge the book. The longer the charge, the more damage. If it's not full damage, it'll still hurt the shrub while also causing a slight push-back, so you can quickly defend yourself in order to get some breathing room without instantly killing a suicider.
To balance this out, suiciders should have a very quick charge (I'm thinking around 2 seconds) until they explode where they move a bit slower and the camera pans out.
Improve respawning:
Currently, respawning is a hindrance. A player just appears at some random point in the map, which gives away suiciders and can lead to spawn camping towards survivors. So, survivors should get a spawn room at a central point, where they would get a few seconds of reduced damage (but not invulnerability, plus they can get knocked back by explosions). Suiciders, when they die, would just 'posses' a shrub that's already on the map, which would be a lot less obvious than a 'pop' in mid-air. Because of this, shrubs on the map would randomly respawn to compensate.
There, that's all for now.
