If they merged all 10 people on shiva with all 8 people on lak there still wouldn't be enough to fill the zone. Relax.
If/When SE incentivizes or forces people to move, I hope they make smart choices for collaboration. What if the only the thing SE changed about Einherjar was that every person in the party gives +1% Ichor? (6-man party, +5% Ichor. 18-man alliance = +17% Ichor)?
...or is that too much?
It's not enough, because botters would rather lose the 11-17% from inviting anyone not in their party than have to pay attention.
I don't know how they fix it. I doubt redoing the entry is feasible, and the way they handled Limbus makes an alternative also seem impractical. Something like a placeholder reservation that has to be open to the public might be an option. IE: one zone of each tier is reserved for the public with a 5 min entry time. NPC hands out the entry lamp, and mobs don't spawn until lamps have been distributed for 5 min. Once the run ends, the 5 min timer starts again. This sort of thing wouldn't require as substantial of systemic changes, though it does require at least new menus and some interactions with reservation system.
Just delete it and add a new ilvl 145 version that has a minimum entry of 3 people and up to 18 people.
No entry requirements, no limit on entry, ichor for every kill, mobs respawn in 60s or less, DI style bosses pop randomly and give everyone on hatelist a key item when killed, move from floor to floor based on your key items and fight Odin with all 9.
Two updates after Einherjar II is released, release a special BCNM that allows you to fight Odin for the Elite Einherjar title because, oops, replacing the entire Einherjar system made that unobtainable.
(Honestly, if you put like a 60-hour reset 60 minute limit on it with 30 to 60 minutes of available time extensions, and had enough spawns for the players who showed up for it, that could be better than Coca-Cola Presents New Limbus. Ein was never much more than "fight the things" anyway. Hell, since the whole connected zone exists dummied out in the files, they could even make use of the space between rooms and make it feel like something brand new.)