Sigh. One of the biggest misconceptions with Hasbro Marvel Legends rears its head again, informed by second or third-hand recitations of pseudo-facts on message boards. There's no embargo on Firestar, no problems with mutants and never has been.
[#1] - Hasbro have never mixed X-Men/Mutants with Avengers in their action figure assortments, because of licensing issues and Marvel politics or personnel. Marvel Legends must thus focus on one property only.
Wrong. Look at every Marvel Universe wave ever (you know, the 3.75" scale line that they actually called Marvel Legends). There was NEVER a time that this line didn't straddle all corners of the Marvel Universe: Fantastic Four, Alpha Flight, Inhumans, Avengers, Defenders, X-Men etc. How could this happen with the alleged rights issue?
Also, look at Black Panther Wave 1 - which mixed up Inhumans, Mutants, MCU & Fantastic Four. Or Red Onslaught/Onslaught. And there are many, many more examples: Cloak & Dagger & Elektra & Daredevil in a Spider-Man assortment. Runaways Nico in a Dr Strange Assortment Captain Britain in an MCU Captain America wave.
[#2] With Marvel Legends, Hasbro have always had character/property exclusive focussed waves - MCU or comic-based Avengers waves would never have other MU properties/characters, likewise Spider-Man & X-Men.
Kind of a variant on #1, but also wrong. As above, you can take Scarlet Witch from the old Odin/Aged Thor wave (a mutant, sometime X-Man etc) and go from there (or go back further to Hasbro's weirdly mixed first waves). One of the best examples: SDCC Book of Vishanti includes key characters from Thor, Dr Strange, Avengers & X-Men, but it's way harder to find a 100% 'pure' Avengers/Spider-Man/X-Men/GotG/Deadpool line than a multi-book/property/license assortment. Black Panther wv 2 & Captain Marvel wv. 1 are possibly the only ones there are (and the latter of those fails on Genis & Grey Gargoyle). Basically, if a character is popular/viable and fits the price point, they're in.
[#3.1] Hasbro don't have the rights for X-Men/Mutants & F4 (film or comic), but [#3.2] now that Disney owns Fox, they can make X/F4 movie-based, or figures from TV shows like Runaways, Gifted, Legion etc.
3.1 is one you hear a lot less nowadays (it would be tough to argue with multiple Deadpool/X-Men/X-Force waves), but Disney's acquisition of Fox has caused some head scratching. To simplify: what is likely to change in the Hasbro Marvel Legends Universe? What will Hasbro start doing that they didn't before? Answer: very little & nothing. They're never going to do a wave of Logan or First Class figures, or FF movie figures. People here may remember the stink of X3 figures like Beast, Juggernaut & Colossus. They tried 'em, learnt lessons and moved on.
So, unless you have full details of the Hasbro/Marvel (Disney) licensing agreement, why go there?
Personally, I'd say Hasbro are covering every niche, nook & cranny of the MU & MCU really, really well. Everyone - myself included - has a wish-list of characters not yet immortalised in plastic (Squirrel Girl is all I'll say here) or teams not yet focussed on (Alpha Flight, Thunderbolts, Defenders) but I'm just going to cross my fingers & wait patiently to see what goodies they throw at me. And, if it's not immediately forthcoming, licensing is usually the last issue that would stop it.