I'm sorry, but I don't find modifiers to relations between countries compelling enough to warrant inclusion of a mechanic of this kind in HOI4.
Consider a few things:
1) Countries join a faction when attacked, regardless of ideology, once the major factions are at war. All the nuance of Falangist Spain in relation to National Socialism in Germany becomes moot if the Allies just DOW Spain. It flips around the other way: all the nuances of the Republicans in Spain become moot if the Republicans win the SCW, but Germany attacks them.
2) Political support for factions is very easy to change right now. Britain or the US can become a fascist haven in a few short years if you put minimal effort into it. Why bother dealing with nuances between liberals and conservatives in Britain and the US, when, as far as the game is concerned, either one being in power is irrelevant to most of the game's rules, and you can press a button and start going fascist immediately.
3) Civil Wars are not interesting enough at the moment to warrant sub ideologies. You can't have situations where there are four or five political factions vying for power in an armed conflict in a country, so why bother tracking sub ideologies?
4) The Axis powers don't really care about ideological nuance in any typical play through. Either you do what the Axis wants, or they invade you. What's that, Salazar? You won't sell me enough tungsten? Time to invade you. No, I don't care about your ideology. What's that, Poland? You're some kind of sub-ideology of fascism? Too bad, I'm running Danzig or War.
If ideology meant more in terms of several other mechanics, like the surrender of Italy, appointment of ministers, and civil wars, then fleshing it out a bit more would be worth the effort. But 90% of the time in an HOI4 game, I don't care about relation numbers between countries.