I was thinking that Niantic can do this to buff Lanturn:
Replace Water Gun with Bubble/Waterfull.
Replace Charge Beam with Spark.
Replace Thunder with Ice Beam.
Bubble or Waterfall + Ice Beam would cover all Lanturns weaknesses. Ice Beam can punish the 5 big grasses quite hard, despite having a low attack stat.
Is Lanturn a viable defender in high turnover areas?
A mini-Vaporeon with electric dual typing. Lanturn has almost as much HP as Vaporeon, but less defense. Like Vaporeon, its fast moves are bad on defense. But its electric dual typing means it doesn't have the weakness to electric that Vaporeon has. Its powerful water charge move Hydro Pump hurts ground types.
So, it has only 1 major weakness, grass. Grass resists all of Lanturn's moves and hits back with SE+STAB damage. But that doesn't make it worse, every Pokemon has weaknesses.
Placing a Lanturn before/after a Dragonite would be a good way to force switches. Dragonite double resists both of Lanturn's weaknesses.
Best defensive moveset would be Charge Beam/Thunderbolt or Hydro Pump. Lanturn has enough bulk to get a Hydro Pump off.
So, I guess Lanturn should be a viable defender.
Edit: Major Text Fixes
Answers
It depends on what your definition of a viable defender is. I remember trying to use these as prestigers in the old gym system against Vaporeons and they just did so little damage due to a low attack stat. They'll take longer to kill but they won't hit back very hard. Personally I like defenders that make the opposition use as many potions as possible. In the current meta anyone trying to take down a gym can do it fairly easily... might as well make them spend as many potions and revives as you can and make them earn it! In the end gym defending isn't that important anymore anyways unless you're in a very high turnover area.
Lanturn's base attack and neutral DPS is somewhere in between Blissey and Lapras.
While it lacks type coverage, it can still take a lot of damage, wasting the attacker's time. Also, the lower CP means less gym badge points for attackers.
Also, fairies are seen rather often in gyms here, but not Lanturns. Many people won't know it has such a high HP.