Make the Night Vision (and of course Eye Ball Mk1) the only mode standard to all mechs, with the option to pick either Zoom, Advanced Zoom or Heat Vision as an additional module.
A less extreme, but also less consequential solution could be making the thermals incompatible with either Zoom mode, any magnification cancelling Heat Vision and vice versa.
The reasoning:
The Heat Vision is currently all too powerful and prevalent, to the point that there is practically no reason to ever switch it off save maybe the occasional visual ID in heavy ECM environments in 8vs8. To name a few specific problems at the moment:
- Spotting and sniping are down to noticing the bright blue dot appear in the distance, and it is practically the same wether the target presents a minimal profile or sits out in the open
- Concealed positions for firing, spotting etc. are non-existent
- Tactical manoeuvres are easily observed, since the tiniest flicker of blue is enough to give away your position
- NVG's are especially useless, since thermals have a longer range and conveniently highlight any enemies
Addendum:
A friend of mine had an interesting idea to complement the less prevalent thermals, by making the TAG highlight a painted target's outline to all teammates. This would in effect turn long range sniping from God eye lonewolfing to sniper/spotter teamwork, at least in cluttered environments, like River City, Frozen City (day) and Forest Colony.
Any discussion, arguments and critique are welcome, but please don't base them solely on such limitation being unrealistic in a 31st century war machine - after all, we are talking about hulking giants weighing the same or little more than main battle tanks of today, firing at each other an arsenal with an effective range of mere hundreds of meters.



















