I'm not too sure what was said on the stream and can't find a specific clip, but as one of the Necromancy designers I'm happy to add some additional context/detail.
The ideal, is that we release a 4th regular spirit as a conjure (e.g. Phantom). For the release, we prioritised getting 3 released and the 4th was a stretch goal. In hindsight we probably shouldn't have added the skill guide entry until we added the 4th spirit.
Balance is definitely something to consider, as adding another regular spirit while keeping the idea of a 4th slot is completely additive, however there are numerous ways we can resolve this so it isn't inherently the blocker.
What we would like to do, is to find an opportunity to add that 4th regular spirit and then explore the concept of 'Hero' and 'Legendary' spirits. I responded to another comment on this thread detailing them a bit more, but the general idea would be that you could use one of these spirits but they would take up more than one of your available conjure slots.
Long term, this might mean that you're running Necromancy builds like:
- 'Skeleton Warrior' (1 slot) + 'Vengeful Ghost' (1 slot) + Dharok (2 slots)
- 'Dharok' (2 slots) + 'Ahrim' (2 slots)
- 'Vorkath' (4 slots)
These are just examples, but wanting to be more transparent about Necromancy thoughts and designs for the future.
Hope that's insightful!