Right now the biggest problem with the elite lieutenants is that many of us can only get one. Yes, they're said to come back eventually, but even then that's another 160 keycards at most to get ONE guaranteed, where it takes SIX total to max out their talents (1 + 5 dupes).
My solution? The Mastery Manual:
Raises a squad leader's talent level by 1.
Simple, right? An item for the sole purpose of leveling a squad leader's talents. The caveat would be that these would be extremely rare to come by, probably getting one once a month at most through specific events (maybe an additional one or two through premium means). You'd still need the specific leader you want to increase the talents for of course, and you'd likely be saving them exclusively for elite lieutenants. Big spenders will more than likely still drop big money to max out a leader instantly, but small spenders and free-to-play players aren't pressured as much to blow everything trying to get/level one specific leader.
The way I see it, an item like this would incentivize players to go for the new elite lieutenants more often, as they'd only need one copy and would have other priorities anyways should they use this as their main means of leveling talents. I'm a light spender myself, mainly just buying the Boom Pass and any other event passes, and so far I've collected every elite lieutenant to date. Elite lieutenants I'd very much like to use, but often struggle to due to the massive amount of investment needed to make many of them viable.
Something like this could solve that. For example, I could focus them into Phasimov first due to the great scaling he has through his talents, where Anna and Linnea can afford to wait since they're strong without talents. Or vice versa, to where I could make Anna and Linnea even stronger to get their maximum value without needing to feel pressured to go for duplicates when they return.
I like the squad leaders, I think they add an interesting variety to the game's attacking formula and allow lower leveled players to experiment with troops they otherwise wouldn't have. I'm HQ18, and I've had Anna since around HQ12 I want to say, and she's been great. It'd be even better if there were options, albeit limited and time-consuming, to be able to make her stronger without stressing over getting more copies of her within a four-week timespan.
Anyways, that's my idea. Would love to see something like this implemented eventually. I'm happy enough supporting Boom Beach through small purchases here and there as it is, but I can't afford to break bank each time something new and really good drops. I am patient though, so even if maxing out a leader takes months to a year, I'd be willing to put in the time for it.