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Serious RPG (Gamemodes)

Ever wanted to feel more pride and accomplishment when destroying hordes of Mental scum?
Or wished that double shotgun didn't suck on final levels of the game?
Or maybe wanted a layer of randomized difficulty in Sam?

All of the above can bring this new gamemode (actually, two gamemodes, one SP and one COOP)!
The mod doesn't replace anything, so it's safe to keep it enabled.

Gamemode adds progression and upgrades to the game. Earn experience by killing monsters, gain levels and Ability/Weapon upgrade points with them.
You can spend these points to upgrade Sam abilities (resistances, ammo gain, etc) and unlock individual weapon upgrades.

Upgrading menu can be opened by pressing "Open upgrade menu" key, which is binded to "U" by default, but can be changed to any other key in game settings.

IF YOU HAVE ANNY ISSUES WITH MOD/MENU BREAKING, PLEASE CLOSE THE GAME AND SEND THE LOG OF THE SESSION:
Navigate to "/Steam/steamapps/common/Serious Sam Fusion 2017/Log/" folder. Open file "Sam2017_Unrestricted.log" with any text editor, copy all text from it and paste it onto pastebin.com. Create the paste and post a link to it in the comments along with the description of the issue.

Gamemode works on all default and custom singleplayer/coop maps, however, because weapon upgrades are manually crafted only for default weapons, custom weapons won't benefit from the system in any way.
Moreover, I recommend against using CWM or RWS mods in this gamemode, as it can screw with its functionality. Avoid using weapon modifying mods in general with this gamemode.
Loot mod should work fine with it, but again, best if you utilize only default weapons of corresponding game.
Also, experience/level gains are balanced (if you can call it that) for default campaigns, so will likely be off on custom maps.

Because you gain advantages by upgrading yourself and your weapons, eventually you become overpowered. To mitigate this, I added Elite enemies to the game.
Every monster (aside from very fat monsters and bosses) has a chance of becoming Elite monster on spawn. This chance is constant, but can be changed in menu using hidden options (see below how to view them).
Elite monsters have more health, and move a bit faster. Additionaly, Elite monsters have 1-4 magic affixes (amount of affixes rises with your level to max 4 at levels 61+). Every affix adds some unique power to the monster. For more details, see here: https://pastebin.com/JfksHQgS
And, of course, Elite monsters award more experience!

You can easily distinguish Elite from usual monsters by their color, and by affixes icons above their head.

Some other important notes:

• It seems that custom difficulties are a bit broken in custom SP gamemodes - "Start" button is not visible in the custom difficulty configuration menu. However, you can still use custom difficulty: while in "Customize difficulty" menu, open game console (by pressing ~), type "gamStart()" (without quotes) there and press "Enter";
  
• For technical reasons, HD rocket jumping had to be emulated via scripts, so you will not get upward velocity if you stand on the floor (need to be at least slightly in the air), and it may not work if you shoot point blank at the wall. But it should work good enough to get secrets and other stuff;
  
• Unfortunately, "Reaper" upgrade doesn't work when the enemy is pierced (like with Cannon or Devastator), because piercing mechanics bug out damage filtering;
  
• Custom singleplayer gamemodes are a bit buggy. One annoying bug is, if you launch any multiplayer game and then will try to start SP Serious RPG gamemode, it will load Classic Co-Op instead for some reason! If this happens, use "Load" or restart Sam to get into singleplayer RPG again;
  
• Another bug of custom singleplayer gamemodes is that they don't load last saved game after your death. So I had to patch it in manually. Every time you die in SP RPG, you will automatically load last saved game shortly, without pressing anything, so don't get confused by this;
  
• If you own JotN DLC and play BFE campaign, after 9th level ("Power of the Underworld") you will play DLC and then resume at "Lost Temples of Nubia", making BFE campaign more whole. Note that you shouldn't skip end cutscene of the last level of the DLC, but watch it until next level starts automatically;

If you are feeling cheaty, or just want to adjust some parameters when playing with the mod, then
hold your "Crouch" key when on main page of the upgrade menu to reveal hidden "host" options. There you can manage players' exp, levels, adjust experience gain multiplier and elite spawn chance.

Credits:
Blizzard Entertainment - some sound effects;
Soulmyr - several visual weapon upgrades; icons for Elite monsters' affixes;
Biomechanoid - "shells on carrier" visual upgrade;

Have fun and report any bugs/suggestions in the comments!