Removing non kingdom troops from the pool won’t do much. Only ultra-rare and epic troops that aren’t in the kingdom will appear in event keys, with the balance severely weighted toward the kingdom troops. It would not improve your chances at pulling the legendary or mythic, and you’ll usually have enough copies of the ultra-rare to make it mythic just by hunting down the legendary. Removing traitstones and filling in the gaps with troop drops (so they drop about 30% more troops per set of keys) would be a start here.
The main problem with this (and every other drop method in this game) is that the absolute need to use “shotgun” tactics in order to get your drop leaves you with a ton of bloat. You can’t target one legendary without targeting the other, all the epics, and ultra-rares at least. And getting something when it is at it’s newest is the cheapest and fastest way to get it. While a lot of us have all the “other” stuff that comes, you don’t at first. The hoard/spluge/shotgun cycle makes it so if you set one goal, you’ve incidentally completed a bunch of others. Want a common ascended to mythic? Well, go ahead and mythic pretty much every common in the game while trying. Want a mythic through VIP keys? Heres a few dozen arcanes and epics and a bunch of legendaries. Crafting would have been a great opportunity to solve this, but the costs and time gates are so severe that you are still better off getting legendaries when they are released by using Event Keys and mythics when they released through VIP keys in the exclusive period - it is still more economical over all, much faster, and you still get the other stuff.
In short, I’d be for them buffing crafting to make it actually usable to grab a legendary you missed in a kingdom event a month or so down after the event has ended. Not an an inflated cost from what you would have had to spend on average, and not overalpping with mythics, and certainly not on a three month wait cycle. Use the jewels as the bottleneck for legendary crafts, make them a one-month cycle for one to appear with a start date of one month after they leave event chests.
I should bring up once again that Event Keys are purchasable in the glory shop at 180 glory apiece in the “spoils of war” pack. We now probably don’t get enough to reliably even get one of each legendary that is released without buying extra event keys now, even by completing several legendary tasks, so keep this in mind. Do not spend your gems on these unless you absolutely have to.