The current state of the Widow Mine has some problems. The first problem is that it's difficult to defuse the mine without detection. Since currently the Widow Mine fires an missile at a target in range that is unable to dodge, the only way to go through the area without detection is to sacrifice one unit. The second problem is that when enemy has detection, mine defusing becomes too easy and Widow Mine becomes so ineffective that barely worth its cost.
In my opinion the first problem is solely due to the way Widow Mine attacks and thus can be completely solved by changing it. The second problem seems a little bit complex, but I think the major issue is that when the current Widow Mine set up to block the area it has to risk the Widow Mine itself to enemy's detection, so when enemy use detection to defuse and kill your Widow Mine, what you lost is not only 75 minerals and 25 gases, but also your time to build units to fill the 2 supplies, which you would have filled with 2/3 of a siege tank. This problem combined with the former problem have created a balance dilemma: the unavoidability and the resulted effectiveness of an undetected Widow Mine means that if the Widow Mine is able to survive the detection some how, it would be totally overpowered. However, this dilemma also means that if there is a solution to the former problem, then combined with the solution of the later problem it will solve the two problem without disturbing the balance.
The following solution to the first problem is based on the fact of the similarity of the Widow Mine's missile attacks and the activated spider mine's attacks if you regard the missile as the spider mine itself. The idea is simple: If instead of shooting unavoidable missile the Widow Mine shoots spider mine that behaves similar to the Brood War counterpart then a lot of issue based on the unavoidability of Widow Mine attacks will be solved in a more interesting manner. Currently those issue are:
Difficulty to fight for map control against Widow Mine early on without mobile detection. If Widow Mine shoot spider mine that is vulnerable to attacks then mine defusing with ranged units would be possible. This however, will not render Widow Mine useless in pro game since difficulty increase dramatically when several mine attacks simultaneously.
Extremely high risk to harass with air unit or drops. If Widow Mine shoot spider mine however, this issue could be solved by adjusting some feature of the spider mine. For example, since spider mine is ground unit it cannot attacks air units directly. It can, however, charge underneath them and explode dealing splash damage. Air units could still dodge it by abusing terrain as spider mine cannot travel across cliff. So again this allows skillful players to dodge mine by micro while Widow Mine has not necessarily lost its anti-air capability.
Although the solution above may decrease the effectiveness of the Widow Mine, with it we could solve the second problem and make Widow Mine more effective in late game while keep its balance undisturbed. The solution of the second problem take one more step further: If the Widow Mine not only shoot spider mine, but set up a spider mine that can attack on its own, then it would no longer requires the Widow Mine to stay at the front and risk itself. Even if the enemy with detection could defuse your spider mine with ease, as long as your Widow Mine itself stay unharmed you could set up another spider mine later. The Widow Mine's ability to set up the spider mine may has a cool down of 40 second, and every Widow Mine might only support one spider mine at a time. The ability could also be energy based, that takes 25 energy which requires 45 second to refuel to set up a spider mine which has a limited lifespan of 45 second. By giving a limited lifespan to the spider mine the Widow Mine is required to constantly replenish the mine field, risking itself, but if you managed to set up the mine field unharmed, the rewording zone control is definitely worth it. The Widow Mine could even has a maximum energy pool of 100 enable it to quickly set a mine field of 4 mines if it have saved enough energy.
With the change above the Widow Mine would have becomes a machine that could set up mine field. What it excels than the previous Widow Mine is that every single one you sneak in and out of their territory without being destroyed gives constant fear to the opponent, even if the mine field has been defused, and stall the enemy and let them wait for detection or defuse with micro one by one. To better fulfill the job the Widow Mine could even have the borrow ability, that allows it to hide in enemy's territory.
So the conclusion is to change the Widow Mine to the following, that both give enemy with sufficient micro to fight it early on, and pose a threat to the enemy even in late game:
Widow mine have 100 maximum energy and 25 initial energy and two abilities: burrow and set up spider mine.
Set up spider mine: it cost 25 energy to set up a spider mine at its location. The spider mine has limited hp (allowing mine defusing) and can exist for 45 second before expired. When an enemy comes in range (such as 5), the spider mine activates and unburrow itself, and after a very small period of time (allowing mine defusing), the mine charges towards the enemy and explode, dealing splash damage. The spider mine can be activated by air unit and charges underneath them and explode, although the mine can't travel across cliff, allowing dodging by air unit. The mine can also be activated by invisible ground enemy if they accidentally stepped on the mine (melee range). The mine has zero collision radius. The Widow Mine cannot set up spider mine while burrowed.
Burrow: Widow Mine can borrow itself and hide from the enemy.