It all boils down to one thing, do you think spell lists are genetically coded in the character?
That is not really so radical, base list means that you need some professional knowledges to learn them successfully with the profession base cost, for that exists the category
other/base, you can learn them, but you have not the basic knowledge so you must pay more.
So, it is not genetic, it is knowledge (less radical as you can see).
Examples, a magician could learn how to manipulate essence into elemental form, a ranger learns how to pray his god to affect narute, etc. This can be a depth knowledge, for that is associated to a profession.
But, reading your doubts, I think your question goes for another way: I am not opposed to allow characters to selects their base list from a bigger group, i.e. a ranger could selects 'the hunter' (channeling companion) instead one of their fixed based. Simply you have the limit of 6 for semi-hybrid and 10 for pure spell users (remember, the 6 + 4 additional for pures).
And, maybe in this way you could allow non-spell users select 1 or 2 base lists and allow to learn them with base cost. But of course it should be well truthful (background option, stay in a magic school some time like a TP, etc.). Think that is the same if you allow a magician buy ranks in weapons with the cost of a thief, it must be really well truthful (like a talent to reduce the weapon cost).
And talking about talents, maybe you could use the one that allows to learn arcane list like own one, but for other/base.