One thing that makes Engy and Spy unique are their PDAs (fourth and fifth slot tools). It gives them a unique role without pigeonholing their main weaponry. Imagine if Spy could only have a revolver, knife and cloak and Engy could only have Construction PDA, Shotgun and Wrench! It would severely limit their role and their combat effectiveness.
I believe the same is true with the Heavy, the Pyro and possibly the Scout, the Sniper and the Medic. Wouldn’t it be amazing if they had an extra ‘PDA’ slot for an extra weapon or consumable? If you look at the TFC classes, most of them have 4 weapons AND 2 grenade-like items.
The two classes that (arguably) need this and should have this the most are the Heavy and Pyro. As of right now the Pyro has to choose between a shotgun and a flare gun. Installing his fourth weapon slot as a flare gun slot would allow Pyro a long-range harassment option (flare gun) and a reliable hitscan weapon against scouts and Pyros. This also gives an excuse for nerfing the Degreaser and the Reserve Shooter.
Adding a fourth weapon slot to the Heavy would also make him much more interesting (at least in my eyes). His fourth slot would obviously be his Lunch Box slot. This isn’t a direct buff for heavies but rather it is a buff for Shotgun Heavies and Beast Heavies (Steak, FoS). With this potential buff to Heavy, Valve could also nerf all the unlocks people dislike (as in just the GRU).
Other classes could become more interesting with this change. Scouts could also get a Lunch Box slot for Bonk! and Crit-a-Cola. This also partially solves the extreme Engi > Scout counter. The Scout can now evade Sentries without sacrificing a secondary. The Sniper could get a fourth slot for passive unlocks or even lunch box items in exchange for nerfing the sniper rifle so it is less effective at point blank range.
I have a fairly complex suggestion regarding Medic’s potential fourth slot. Imagine if you could mix ‘backpacks’ with mediguns to get slightly varied medigun stats. Basically the Medigun would control traits like overheal rate and max overheal while the backpack dictates what kind of Ubercharge the medigun has. I could explain this idea in greater detail here but I think I will save it for another article. An alternative suggestion for the Medic is that he simply gets an additional weapon slot. Perhaps the pistol could fit here.
Soldier and Demoman are left out of this because they are already powerful and versatile enough as it is now. The Demoman would be unbalanced if he had a stickybomb launcher AND shield so a fourth slot for him is unlikely. The Soldier’s fourth slot would likely be a banner slot although I think that would make Soldier VERY powerful. Imagine a roamer soldier WITH a charged banner!
While this idea seems hard to balance and implement I think it would be definitely be worth the effort!