If I'm proficient with a greatclub, I could use a greatsword as one and get proficiency. That's without a feat.
I'm bemused by your interpretation of the improvised weapons rules, and this example is the reason why. It seems to me you're interpreting the rules as "If I use a weapon as another weapon, it becomes an improvised weapon" where personally I see it as "If there's a
nonweapon object that I use as a weapon that resembles a weapon, for all intents and purposes
it is that weapon".
If you're using a greatsword but aren't proficient with it, you aren't able to say "Since I'm proficient with a greatclub but not a greatsword, I should be able to treat this sword as a club and gain proficiency". Treating the greatsword as a greatclub is a great example of why you'd use it as a greatsword (even if you were arguing it's an improvised weapon, it curiously resembles a greatsword and therefore
is a greatsword) but not add your proficiency modifier (if you aren't proficient), because you're using it the same way you'd use a greatclub, while someone proficient with it would know the proper way to use it.
Players who pick up an unfamiliar weapon, without any feats or proficiency, have the choice of using the weapon as something they are proficient in (and thus losing access to whatever special features the real weapon has that the thing they're using it as does not - for instance, you could use a halberd as a greatclub, but you would presumably lose the benefit of reach while doing so. With multiple attacks, you could treat the weapon as a greatclub (and get proficiency) in one attack and as a halberd (and get reach, but no proficiency) in another. That's all fine by the basic rules.
This is another example of our differing opinions, and your example, again, is one that I find baffling. You should only have the option of using the weapon as one you're proficient in
if the use makes sense for the weapon. A halberd is a head at the end of a long pole -- if you're using the head and arguing it's a greatclub (for whatever reason), the "improvised use" of the weapon makes it resemble a halberd, so it becomes a halberd rather than a greatclub. If you switch the end and argue that you're using it as a greatclub, it more closely resembles a quarterstaff anyway! A long, relatively thin pole used as a long, relatively thin pole makes more sense than using it as a slightly shorter and unevenly thick weighted club... Especially because it would be weighted on the wrong end.