What concessions did Hitler demand from the Poles before 1939?

On August 31, 1939 Reich's Foreign Ministry sent and ultimatum to the Polish Government, a so-called "16 points ultimatum".

This is basically a reiteration and expansion on the earlier demands, but they have to be considered in the broader context. The Non-Aggression treaty of 1934 between Germany and Poland has been terminated. There are provocations almost daily on the border with Poland, with occasional shootouts and casualties, mostly on the Polish side (Gliwice Radio incident not withstanding). Germany Annexed Austria and Czechoslovakia. There is German-Soviet Treaty published. General feeling in Poland is that Hitler is basically not to be trusted. Which means that whatever the Germany proposes is either a bad-faith deal or straight up-ruse.

Add to that that Poland has literally THE best intelligence network in Reich . Arguably, it was the best in whole of Europe, even after the fall of Poland (and regardless of big error of Polish Intelligence, which allowed for part of it's archives to fall into German hands, with results of losing about 100 of agents across the Reich). This means that Polish Government has a very good general idea of the German plans.

So the Ultimatum is relatively benign, but that's until mid-1940. What's next?

It is very important to note, though, that there is a preamble to this Ultimatum, which basically negates the Versaille Treaty, which also means Polish State becomes unlawful, in Reich's view. Of course, it's not stated explicitly, but you judge for yourself:

The situation between the German Reich and Poland is now of such a nature that any further incident may lead to an explosion in the ranks of troops occupying positions on both sides. Any peace solution must be so structured that, the next time the circumstances give rise to this state of affairs, they cannot be repeated, and therefore not only Eastern Europe but also other areas will not be put under the same stress. The reasons for this development lie:

1. In the impossibility of setting out the borders, as delineated by the Versailles dictate.
2. In the impossibility of treating minorities in lost areas.

The government of the German Reich, therefore, in these proposals assumes that a final solution must be found, that will remove the impossible situation related to the demarcation of borders, provide both sides with communication lines that are vital for them, and the minority problem - if at all possible - will be eliminated or, if that is not possible, it will make bearable the fate of minorities by fully guaranteeing their rights.

1. Free City of Gdansk is returned to the Reich based on the unanimity of it's pure German population.
2. The so-called Corridor from the Baltic to the line of Kwidzyn - Grudziądz - Chełmno - Bydgoszcz (icnluded) will decide on it's own on it's joining Germany or Poland
3. Which will be carried out by vote. All persons who lived there before 11.11.1918 and expelled afterwards will return for the purpose of the voting. Vote will be guaranteed by Commission formed from citizens of four countries: Italy, USSR, France and England, which will have sovereign power. All Polish officials, civilian or military and Polish police and military must be removed.
4. Polish port of Gdynia is excepted. Exact borders of this Polish town will be decided by Polish and German Governments or international mediation.
5. Said vote will not take place before 12 month period.
6. In order to guarantee an unlimited connection for Germany to East Prussia and Poland to the sea during this period, roads and railways will be designated which will enable free transit. At the same time, it will be allowed to collect only those fees that are necessary to maintain communication links or carry out transports.
7. A simple majority of votes cast will decide on the area's belonging.
8. In order to ensure free communication from Germany with their province of Gdańsk-East Prussia, and Poland with a connection to the sea after the vote - no matter it's outcome - there will be an extraterritorial communication zone for Germany, if the plebiscite area goes to Poland, more or less in the direction of Bytów - Gdańsk or Tczew, suitable for the construction of a motorway and a four-track railway line. The construction of the motorway and railways will be carried out in such a way that Polish communication lines will not be affected by it, i.e. the construction will pass either over or under them. The width of this zone is defined as 1 km, and the zone itself will territorially belong to the Reich. In the event that the vote is in favor of Germany, Poland, for a free and unlimited connection to its port of Gdynia, will be granted the same right of the same extraterritorial road or rail route as Germany would have.
9. Should the Corridor fall to Germany again, the Reich states its readiness to carry out a population exchange with Poland in the size corresponding to the Corridor.
10. Eventual special powers in Gdańsk postulated by Poland will be granted in exchange for identical German powers in Gdynia
11. In order to eliminate any sense of danger on both sides, Gdańsk and Gdynia will be given the character of purely commercial cities, i.e. without military facilities and fortifications.
12. The Hel Peninsula, which either goes to Poland or Germany according to the vote, will in any case be demilitarized in the same way.
13. Since the German Government has the strongest accusations to bring against the Polish treatment of minorities, and the Polish Government, for its part, considers that it must also make accusations against Germany, both sides agree that these complaints will be submitted to an international commission of inquiry, task of investigating all complaints of economic and physical damage and other acts of terrorism. Germany and Poland undertake to compensate both minorities for all economic and other damages caused since 1918, or to cancel all expropriations, or to award the aggrieved parties full compensation for some kind of violation of economic life.
14. In order to deprive Germans staying in Poland and Poles staying in Germany of the feeling that they are not protected by international law and, above all, to ensure that they will not be forced to act or duties inconsistent with their national sense, Germany and Poland agree that the rights of minorities on both sides must be guaranteed through extensive and binding agreements in order to ensure the preservation, free development and activity of their nationalities by these minorities, allowing them, in particular, to organize themselves for this purpose. Both parties undertake not to appoint members of the minority to military service.
15. In the event of an agreement on the basis of these proposals, Germany and Poland express their readiness to order and carry out the immediate demobilization of their armed forces.
16. The further steps necessary to carry out these activities will be determined jointly by Germany and Poland

Source: Ultimatum niemieckie wobec Polski (tzw. 16 punktów) [Polish only] (1)

(1) - Translation is mine, all rights reserved.

There is an explanation for a second question: Why Poland thought it would threaten their independence?

As foreign minister Józef Beck said 5 may 1939:

„My w Polsce nie znamy pojęcia pokoju za wszelka cenę. Jest jedna rzecz w życiu narodów i państw, która jest bezcenna: ta rzeczą jest honor"

Roughly translated:

In Poland, we ignore the idea of "peace at all cost". There is one thing in the lives of nations and countries which is priceless; That thing is honor.

Or as the field marshal Edward Śmigły-Rydz said:

Jeśli ktoś w kraju liczy na jakieś chwile słabości, to jeszcze raz nieudolnie się przeliczył. A jeśli ktoś z zewnątrz na taką okazję kalkuluje, to niech wie, że my po cudze rąk nie wyciągamy, ale swego nie damy. Nie tylko nie damy całej sukni, ale nawet guzika nie damy od niej. I niech wie, że to jest decyzja całego narodu

That would be:

If anyone in Poland counts on our weakness, he is sorely mistaken. And if anyone abroad plans for such an opportunity, let him know that we don't reach for what doesn't belong to us, but we won't hand over what is ours. We not only won't hand over a gown, but even not even a single button. And let him know that's the decision of the whole nation.

To sum up: Giving anything to Germany was not an option for polish diplomats at this time. They were not afraid to fight, and they even expected to win. For their miscalculations, Poland paid dearly.