Firstly, Fearless actually isn't very good. Why, because it has extreme antisynergy with Courage of the Colossus, which is the Keystone ~80% of the people who'd even look at Fearless would run. When you proc Fearless and CotC at the same time, which happens a lot you wind up providing extra armour and MR to your shield which is already a strong burst deterrent. The enemy is likely to either wait for CotC to fall off or DPS through it, in either case Fearless procing isn't that useful. The only champs I could see this being useful on are burst prone bruisers like Irelia or Fiora, but even then, Insight is so good.
There's no way to make Perseverance anywhere near as strong as Insight without flat out breaking Perseverance or dumpstering Insight. Same goes for Fearless to a lesser extent, you probably could make a raw stat mastery that competes with Insight, but it would have to be far less situational and very strong. Something like 'gain 7% A/MR unconditionally' would be competitive with Insight, the problem here is that we're talking about the strongest 4th tier mastery in the game, so we need some broken shit to level out with it.
IMO what Riot should do is move Insight to T5 alongside Swiftness, which is another disproportionately strong mastery, Insight is probably still far better, but with small buffs there'd at least be situations to run Swiftness. Currently if you spec 12 into Resolve you take Insight 100% of the time, it's just better than the other masteries.
From there I'd look to balance out the other T4s with Fearless, which would mean buffs to Perseverance and most likely a new defensive mastery more for tanks, since Fearless is actually, somewhat counterintuitively most valuable on medium durability bruisers like Irelia even with the 10% boost, the flat just matters more.
I'd also look to buff some of the incredibly weak early masteries in the tree cough Veteran Scars cough. Especially so since moving Insight to T5 with no other changes would make 12 in Resolve super weak. Currently if you're 12 in Resolve you're doing it almost entirely for Insight, some champs like Annie literally go 12 in Resolve for Insight, they spend 11 points on trash they don't care about because of how insane this mastery is.