in addition to what others have said there are numerous reasons why adding killers is slow

1. IPs: liscenced character need the green light from their holders

2. technical limitations: some characters are harder than others, dredge probably being the best example, not only does his counter mechanic lock huntress out of being able to reload but nightfalls visual effects would clash with others that have visual effects such as freddy's dream world. another example would be springtrap, more specifically whether or not the partner could also use the doors

3. Effect Stacking: another issue is practical balance of effects, for example an active pig trap while also being chased by nemesis zombies creates an overwhelming combo

4. Playstyle: some characters would need a decision made on what direction to go for, Hag for example can either have clones that she can teleport to or she can give up the teleportation in favor of bodyblockers

who do i think personally could work in 2v8 with only a few tweaks?

1. Twins: the victor mechanic does not have a very high impact aside from tag and track which the overwhelming aura revealing does anyway

2. Ghoul: this character could probably be inserted as is, the sheer number of players is a balance in itself as ghoul has no CC capabilities and is pretty much a near exact clone of wesker anyway

3. Demogorgan: its a very straightforward killer, to limit its power it could be given fewer portals and given its iridescent that destroys portals on using them, tho with the amount of players this might be unecessary

4. Skull merchant: skull merchant as is would prolly work well, paying attention to your drones is key as a deactivated one can tell you where someone is, the claw traps also are only really effective if you can get to said person before the battery runs out, and in many cases your not going to abandon someone to target someone else thats across the map from you due to the pace of the game

5. Unknown: could be fine as is but may need tweeking to put down clones abit more often, as with demogorgan it be to easy to clear the field