Instead, how about giving water\lava cells the ability to turn back into empty ones? [by placing water\lava source blocks]
Well sure, being able to place water (and by extension lava) with cells would work too, and probably make just as much sense.
First: two buckets of water enough to fill any volume.
Second: there is a mod for such buckets.
Third: doubt tin cells will be made reusable.
1: Sure two buckets of water can make an infinite water source (I have several), it's just that I have to keep refilling the buckets to make my lake. Making any sort of large water feature would be just as annoying to make with buckets that you have keep refilling and refilling.
2: Just because there is another mod for it doesn't mean it's a bad idea for this mod. And on the vanilla MC wiki they don't like people just saying "there is a mod for that", I see no reason why it would be any different here.
3: It wasn't my idea to make cells reusable, although it's a good idea. My idea was to make a hand tool out of cells that is reusable.