Level Up (A5E) Multiclass Subclasses

[The Crimson Binome]

That's funny how the eldritch knight had, indeed, the same level progression as a lvl 7 wizard. Never noticed that.
When you would gain another class level instead of your archetype feature, would you use your class HD value for HP or the multiclass class' HD?

For the sake of simplicity, I think it would make more sense to retain your base class hit dice for all levels. This isn't true multi-classing, after all. You're mainly a fighter with some lower-level wizard abilities, so you should still be a fighter in every way that counts.

From a balance perspective, it seems like it would strongly promote certain combinations rather than others; and access to low-level wizard spells is certainly not sufficient to justify a fighter being less capable of taking a hit. They aren't going to spend a third of their combat actions to cast spells from the back line, or anything.