A gamemode which would be interesting to see is a slayer or infection where you get stronger or have certain abilities the more kills you get. Every “peak” you get a new buff to your character which can be increased speed, damage, health or shield resistance, increased regeneration, more loadouts, etc. Here’s a diagram.
10 Kills - +25% Damage
15 Kills - +25 Speed, 10% Health Regeneration, New Loadout
25 Kills - +25% MORE Damage
30 Kills - New Loadout, +25 Shield Increase
and so on…
But it will vary so you can customize at how many kills you need for certain buffs and also customize what types of buffs are included…