Better solution. 4 slots for "conjures" but each conjure has a "space" value. So say you have Dharok introduced as another conjure. He would hold like 2 spaces instead of 1 so when you conjure army they would be able to do 3 total but balanced around space total.
This has actually always been an intention of ours!
In the original design we discussed having regular spirits as the base conjures (i.e. Skeleton, Zombie, Ghost and Phantom) and then gaining access to 'Heroes' or 'Legendary' spirits (e.g. Barrows brothers, Ancient warriors, maybe even Vorkath) from other content which would use up 1-4 slots.
We haven't spoken too much about ideas that are on paper for Necromancy in the future, but this is just one of them. You'll find that most/all projects have a ton of ideas that don't come to fruition for the original release, but are written down to tackle and explore in the future.