Rather than “take toys from someone else”, why not ask GW for more unique toys for more unique boys? I know that’s what I’m going to be doing the next time they give us one of those “serious community surveys”.
The Marines travesty is that GW released six pre-9th Marine supplements, and out of approximately 380 pages, we got three new characters and one new unit. The Raven Guard, White Scars, and Ultramarines didn’t even get new characters.
All of the unique gene-lines should have gotten some unique units, whether they were some special bike units for the White Scars, some new type of Sneaky-Beakie Vanguard/special ops type unit for the Raven Guard (or even crazier, why not an over the top shadow assault close combat unit), some heavy hammer and anvil unit for the Iron Hands, and some kind of unique siege tank or bastion line element for the Imperial Fists. Heck, the IF or UM could get a shield wall type unit and it could be made to fit a theme for either.
And we may get any of these in a 9th Edition supplement for these gene-lines, but as of right now, anything not BT, BA, DA, or SW is just left in a sub-par place, which does stink - the answer isn’t just “take unique stuff from those and redistribute” though. The BT seem like they might be the most unique out of anything Primaris, that’s for sure.