When I think of GCIV, I wonder what should be changed to make it a new game and not just an expansion of GCIII. Don't get me wrong, I think it should expand on what is best in GCIII, but my thoughts go to what makes it a newer, better game.
And in that, I generally agree with the OP here.
1) Change how the map functions.
The entire feel--and strategic depth--of the game could be improved with changes to how the map works. More than just distances that separate, I'd like to see a greater level of player vs. environment and a strategic use of the environment.
3) Diplomacy in most 4Xs is a extremely simple and largely risk-free mini-game. Make diplomacy a higher-stakes activity, with the potential for failure.
Also, what about ongoing negotiations? Counter offers? Make diplomacy not just a screen the player opens, but a vibrant part of the game that carries strategic importance and risk.
4) Combat needs a serious overhaul and serious attention this time.
So much could be said about this, and I'm sure everybody has an opinion... for me, I don't really want any combat micromanagement, but I want a robust system that requires some macro-level strategic decision making... a system that makes all hull types valuable in certain circumstances; a system that doesn't result in 100% victory or 100% loss; a system that allows ships to be disabled and potentially captured; a system that allows the player to use the galactic geography to his or her advantage; a system that allows more than just peace or war (e.g., seizing ships that violate your territory, establishing blockades, etc.)... I would love to see a system emerge in which everyone argues about what works best and no one is 100% correct because it is so nuanced. Whatever form individual battles end up taking (I know there is a discussion taking place about some level of control of individual fleets/battles), my hope is that players are given more strategic choices in ship design, fleet composition, and "policy".
5) Have mod support
In general, mod support and documentation is fairly poor in GCIII (in my opinion). What can and cannot be done is opaque, and figuring things out requires a lot of trial and error (and a high degree of patience). I've all but given up modding because I've found less and less enjoyment in it (and I have less time for it)--and I'm in some ways surprised with how much Gauntlet and Horemvore, among others, have stuck with it... between the game's constant, sometime major changes, the virtually nonexistent support, and the significant limitations. For GCIV, I hope data structure/management and modding is given a lot more careful planning up front to avoid a lot of the issues in GCIII.
First I think the tech trees could have done better.
I agree with this, too. Overall, the approach is good, and the current tech tree iteration does offer meaningful choices. But I do see an opportunity to do something new and better. I don't like specializations that remove choices (why can I not research how to colonize frozen worlds after I've learned how to live on barren wastelands?), but I do like the idea of focusing research on certain priorities that, by focusing on those priorities, preclude a focus on other priorities. For example, focusing on capacity rather than hull strength.
Perhaps instead of only researching individual technologies, perhaps more could be done with the concept of "capstone research"--allowing players to spend time researching certain focus areas and increasing bonuses to those focuses (e.g., instead of researching a specialization in hull capacity that results in +10% and leads to the next tech, researching hull capacity for a number of turns that allows capacity to get better by some small percentage per turn). That's just one idea among many possibilities of how to transform the tech tree from something that feels generic--even though it has merits--into something new and with new potential. And maybe that idea is bunk. But taking a fresh look at how research works would be good, I think.
I'll add that the concept of ideology could use an overhaul (or done away with). I think choices made by players (and AI) should reflect faction personality, which may be a mix of things. When given the option to use military force, some civilizations will do that always. Others might do that under the right circumstances. Others might never use military force apart from defense. Making ideology more nuanced could tremendously impact many areas of the game--from colonization to combat. Instead of arbitrary rewards for accumulating points in a category, why not make ideology about pursuing goals--undertaking massive projects that reflect a faction's underlying personality. Maybe once you acquire a certain number of points in a category, special projects, citizen types, research/techs, governments, etc., become available. This is present in a small way in GCIII, but I think this could be taken further... Just thinking out loud on this--I'm sure there are many great ideas on how to make "ideology" and/or faction "personality" a better game mechanic than what currently exists.
That's it for now... I've spent a lot of time enjoying GCIII, and I hope that the development team for GCIV keeps much of what makes GCIII so good, improves on that, and turns the few things that don't work as well into something much better.