If someone wants to make a mage/ranger hybrid it should be possible, and can be easily balanced. Think about 100% Warrior getting 1.8x multiplier to strength and stamina, while 50% mage and 50% archer could get like 1.55x dexterity multiplier, 1.25x magicka multiplier and a 1.1x stamina multiplier. Also, the most powerful abilities would only be avalaible for those who put at least 75% of their points into the hypothetical main tree(or however else the customization is handled).