Hello
I play a six keyed M&E flute and want to start using my keyes more. I also play the boehm flute, especially Telemann and Vivaldi are stuff I used to play a lot. These days I don’t play it much anymore, because of my passion for the simple system flute and ITM. However I want to use my keyes more, Telemann Sonatas are somewhat a nice opportunity for that but when I try to play them I always have in mind how fluent I played them on the boehm flute…and that sucks. So I want more of a reboot in terms of repertoire.
I am searching for a compact collection of repertoire, that is playable on the simple system. Like a fiddle tune collection or so. It is often a bit overwhelming to look for some tunes of different origins/ traditions, because there is so much out there…thats why I would prefer to find a more compact collection of tunes, like a written down tune collection that I can work through. I hope my intention is not to diffuse 
I am generally interested in various european folk music and baroque, old time fiddle music seems also interesting. But in general I just search for a nice source of non Irish repertoire that still fits my flute and demands my to use my keys more often 
It sounds like you’re describing the John Playford Collection. His tunes come from the Telemann era and sound great on a wooden flute. A frustrating number of them are in C, F and Bb, giving you plenty of opportunity to work your keys. This repertoire is sort of half way between Telemann and ITM - a baroque folk-dance tradition. You can throw in all the ITM ornaments you please. In fact, you’d better. This music sounds awful if you play it stiff off the page the way many conservatory musicians do.
Here are some Swedish sackpipa tunes. Most are a good workout for the G# key.
http://olle.gallmo.se/sackpipa/music.php?lang=en
In addition to the excellent suggestions above, check out this page on Terry McGee’s website: http://www.mcgee-flutes.com/Learning-the-keys.htm
As you might imagine, this topic is scarcely a new one and Terry’s page grew out of discussions here and elsewhere.
If you peruse my YouTube channel and the resources linked in my signature below, you’ll find quite few tunes from various traditions which are in key-needing tonalities. There are quite a few in the Welsh tradition.
I’d also say, in response to your comment about “smoothness” playing baroque material on Böhm flute, that that is in fact something you need to get away from - it’s one reason why baroque music tends to sound bland on modern flute, especially compared to traverso. Of course you need to gain sufficiently fluid mastery of the simple system, but that is simply (!!!) an issue of practice. Playing the mainstream baroque repertory (Bach, Telemann, Handel etc.) on 8-key is a very good way of enjoyably working for that familiarity.
You don’t need to venture away from Irish trad to use your keys, as there are plenty of tunes that make use of them in Irish music. Scottish music has plenty that would make use of them, as well. Pick up tunes from fiddlers and you might be more likely to come across them more.
In my opinion, it seems a bit strange to specifically look for tunes that use keys.
In any case, a lot of Swedish tunes make good use of them, and many such tunes suit simple-system flute really well, in my opinion. Until recently I played Swedish tunes almost solely on Cittern, but as of late I’ve been transferring many tunes I play on to the flute, and it has been good fun. Of course, it is best to pick it up from players themselves , however this website has a huge collection of various Swedish tunes.
This takes you to a page with the list of different types of tunes, and under each link is a list of various tunes, available in sheet music and ABC.
If you don’t want to take the obvious (but challenging and rewarding) route of playing early 19th century study material that was actually composed for the 6-8 keyed flute, there are countless English sources of traditional dance music that cover a good range of keys. Many of them are fiddler’s manuscripts that can be got online from the likes of the EFDSS or the Village Music Project. The peerless Playford has been mentioned but for a later repertoire, a good introduction might be the two Boldwood Dancing Master books - compiled by the group of that name - that are available online at http://www.boldwood.org/. A good variety of tunes from various (mainly late 18thC) sources and lots of lovely flat keys to get your fingers and brain working in tandem (as mine tend not to do). Some of the tunes are taken from publications that were aimed in part at the large number of amateur flautists in England at this time.
One philosophical issue might be how appropriate it is to try and play such material with latter-day Irish-style embellishments, undoubtedly impressive and effective though this can be. If I had a bit more moral fibre, I might try and initiate a thread titled Towards an English Traditional Flute Style. But I want to get out of here in one piece…
Oh, g’wan! 
Coincidentally I just did this the other day, after a rather English encounter at Shrewsbury Folk Festival the other week. https://youtu.be/L8PfPSsgPao
Come on, Will, do it. I peeked behind the green curtain a while back with a thread on Scottish fluters and there were no cries of “Heresy” or villagers with torches. I’d be interested to see what it elicits.
Best wishes.
Steve