Well, one could change values as health, mana or whatever else you got, and freeze that. Making yourself invisible, it's possible if let's say you got a skill that makes you invisible (or whatever you want), then you can freeze it to always state it's on. Bit of work for it, if it has a long cd but if you take the time, I'm sure you'll get the result you want.
Same thing with Rapidfire, unlimited ammo is way easier though, cause the ammo isn't a state of on/off but a value that goes down to 0 then reloads, unless you have the option to reload before that.
Basically, you can do anything you like to, as long as the app values isn't on a server.
Modding the APK is a possible way, if you know what to do, it might even take less time and be a permanent fix to it as well.
What is the game you want to do this on? Sure if you posted a request, someone would mod it for you.
//Moods