Alright, let me try to address this as three seperate (altough possibly mutually-exacerbated) problems. These are not just things you mentioned in your post, but also what other people reported in the past regarding difficulty.
1. Characters hit level cap too soon.
This will in part be addressed by veteran levels. Once you hit level 25, the progress slows significantly and you are still able to increase the power, keeping the leveling process meaningful.
2. Game is generally not that challenging past the mid-point.
I understand this is often the case and we are considering how we can adjust certain encounters and areas to provide more challenging experience for players.
One of the main issue here, we think is the absence of high end non-human enemies. Most of these types of enemies appear in the late-early game and mid-game and are carried over to near-endgame where they are simply not that challenging anymore.
In addition to this there is a rollercoaster difficulty ride with the late-midgame to endgame encounters where some are too hard even on the easiest difficulty, while a lot of them are too easy even on the hardest difficulty. In the future, we will be looking at those encounters separately and work on tweaking them for various difficulties.
Which leads me to the next point.
3. The game needs another difficulty setting.
Some of you RPG veterans just need a space of your own when it comes to difficulty.
4. Energy shields.
I don't want to get into this right now, but personally I am not content with how the shields work at the moment, both in the terms of crafting (where there's effectively one way to go about things) and with the amount of defense they provide with practically no active tactical or strategic engagement from the player and virtually no downsides.
Expect this to change at some point (prior to the end of the world).
5. Expedition DLC will increase your power before a portion of original campaign and thus make you out-scale the content.
The content of the DLC occupies the are from the mid-game to late-game which means that the leveling advantage it provides need only be accounted for in the late-late-game and the endgame itself and this it will be doing.
There is no point attempting to adjust the mid-game content towards it as we do not do this for other horizontal mid-game stuff - Foundry, Rail Crossing, Core City, Drones/Protectorate, etc.
6. The problem of classic XP.
While it will take a lot of experience to gain veteran levels in classic XP mode, I don't really have a good way to control the levelling curve here due to the non-linear nature of the large portion of the game.
Actually, I did think of a good way of controlling it and I implemented it in the form of oddities. If it was up to me, I would just do away with the classic experience, but I am obliged to keep it due to the late date in which the oddity addition was made.
I have no good solution to this right now, I'll have to think about it.