Wouldn´t change a single major thing when it comes to Cyseal since I remember it as an all around excellent experience.
Afterwards, though, the winter land excepted, the game went steadily (and quite sharply) downhill so what I hope for is a big overhaul of the game´s second and third acts.
The aspects I hope will improve the most are writing, encounter design and, to a slightly lesser degree, quest design and puzzles.
writing - while I don´t think D:OS pulled off the humorous stuff as successfully as DKS and perhaps went a bit overboard with the frequency, I still found it entertaining for most of the time and the generally lighter tone is something I enjoyed the game for, it´s mostly the serious moments, including pretty much the whole last 1/3 of the main story, which I remember as leaving a lot to be desired, and the joinable companions whose writing felt like an afterthought (haven´t played with the later added ones though)
encounter design - less encounters, more uniqueness please
quest design - don´t remember much details, but I do remember wishing for more C&C in general (in particular when it comes to the last act´s town which I remember as a missed opportunity in this regard) and more involvement of NPC dialogues in quest solutions (particularly in act 2 where the NPC aspect didn´t feel appropriately fleshed out)
puzzles - don´t remember much details here either, but better telegraphing in some places and less reliance on pixel hunting in general would be nice
When it comes to act-independent stuff, I really hope the character system gets infused with a wealth of high level abilities (ideally coming hand-in-hand with encounter design accounting for them) since these were almost non-existent in the original. I´d also certainly welcome a significant increase of hand placed loot.
As a minor thing, I´d like combat tracks to be picked based on relative difficulty of encounters, at least when it comes to one particular bombastic track pulled from DKS which felt wholly inappropriate when played during easy encounters
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There are also a few things I hope won´t change much - I´ve found the game´s combat system to be pretty much the best turn based iteration in cRPG to date so I hope there´s no radical change to its basics and I also hope the enhanced version in general remains as refreshingly handholding-free as the original.