Slitherine

I played the regular and Afrika Korps on Colonel, went on with West-Campaign DLC on General and now finished East-Campaign DLC on Field-Marshall level.
Although I met my Waterloo at Berlin, which is historically correct .

In the meantime I have aged two years (53 now), so my compliments to the designers and modders for keeping me off the streets for such a long period...
PC has everything you need for strategic players as well as players that just love doing the maths and statistics.

So obvious question - whats next when wanting to have the game more challenging...

Wish to continue playing German side, so wish to start Eastern Campain all over, with 1935-37 MOD.

Could go to Manstein/Rommel/Guderian level, but have played with rules 1.14 so far, so can also change to 1.20.

But if there is one MOD I would applaud, then its the ability to check on/off these 1.20 options, instead of all or nothing.
For instance:
Agree with higher price for additional 11, 12 etc units, (of course) also with points gained when AI unit surrenders.
But I definately disagree with losing +points when upgrading unit outside of "family". Also the fact that disbanding an experienced units gives you equal amount of points back as an green unit would cost (though not 1.20 rule).
I know I will also need another good look at the soft cap rules, which seem so very confusing and in my opinion lean too much on the statistic abilities of a player, instead of a good balanced mix of strategy/statistics.
Having reached Berlin I know also know what it takes to get that far: loads of Tiger II's, anything else just gets smashed after 1944.

Will have another go at Berlin in the meantime, this time trying to focus more on attack/defend in south, and only defense in the north and only focussing on maintaining control of both southern airfields.

Anyone know if there's a MOD that continues after April 45, with a "fresh" German attack on Russia, incorporating the "what if" armor and planes from both sides?

Well, there is a few DLC available which you didn't mention playing.

soviet corps - it's nice to play soviet side and it's quite challenging.
allied corps - again it's nice to play some allied gear.
US GC (3 in total) - this is nice and have some fresh stuff to do. Quite challenging as German have strong gear and facing them is very tough.

I do hope that there is a GC for soviet side soon and also extend to pacific theater.

Also I would recommend that you check out Order of Battle Paciific, it's kinda like PcZ with different mechanic and it's Pacific theater. I found both games are nice to play.

For myself I keep returning to Panzer Corps because I am trying to achieve all achievement. Still halfway thru Afrika Korps using Italian units only, it was delayed because of US GC release. Also I found myself going back to GC39 and replay it all the time trying different style and tactics.

Soft cap rules it will change the entire game as there is no indicator so manual monitoring is needed and it will be a totally new challenge for the entire campaign. It's confusing if you are first time using soft cap rules but it is really fun using it which I enjoy a lot.

I'm enjoying US Corps mostly because there's some interesting force balances to get right, for example what is the ideal tank to tank destroyer ratio? You need the Shermans for flattening soft targets but you need tank destroyers or you can't scratch the German tanks. You can develop the halftrack AT to gain experience quickly as artillery, but their armour is about as effective as cardboard. Even the armour on the M10 and Hellcat is not very good, so you are going to have to accept a lot of damage and learn how to manage damaged units in a crisis. First time round I also found out that it's very easy to run out of prestige, so you need to plan ahead.

Then there's the mods, specific challenges (historic mixes, reduced prestige, nominate one unit as your command and surrender if it gets destroyed and others).