You should get slowness after 1 min of holding back a regeneration that will increase more every minute. Also after abt 2 mins you should get blindness 1, then once you get to 4 mins you get nausea until about 20 seconds before regenerating. 

I also think it would be cool to add a feature where you can let a little energy out to reduce the effects, making your screen glow like when you're about to regenerate, that releases some energy increasing mob spawns (Like they came because of the energy release)

Also killing every mob your cross-hair is aiming at in an energy burst that can be engaged at the cost of one less minute of delay of the regen, similar to The Time of The Doctor, when the Doctor releases regen energy, killing the Daleks.

There should be a new setting in river song's diary that allows you change the regeneration animation to look like the first doctor's regeneration or the forth doctor's regeneration with the watcher appearing