At first glance of Shadow Fiends talent tree, it is clear that the DOTA Devs have tried to add a 'Magic Damage' path to follow, similar to Ember Spirit.

These include:
Level 15: +6% Spell Amplification
Level 25: +150 Shadowraze Damage

The issue with this path is that it's terrible.
I would like to suggest a more persuasive level 10 talent such as "+15 Intelligence" and a level 20 talent "+15% spell Lifesteal".

Shadow Fiend is only ever played one specific way, and always has. I think some changes like this would add a fun change to the hero, and would convince players to experiment with new item builds.