When you've got a chance to try something this unique you've got to take a chance. Not many developers are this free of publishers prodding you to release. Go for the longer development cycle.
The only thing I would suggest as far as the schedule is concerned is to keep the release date the same, but decrease the time between all of the initial phases. The hardest problems to fix with games aren't usually with the individual components (world map, tactical combat), but with how they relate to each other and how balanced they are. I'd rather see more unpolished components together early on to get a better picture of how things could/should work, then put the final piece on later in the cycle and then not have enough time to balance the product as a whole. We might also think something like the world map is working great and then near release we find that the whole game warrants a feature that greatly changes the world map.
How many players should/can we allow in a game? 8? 12? 32?
I can't see players playing with more than 8 in a multiplayer game, but there will be singleplayer people who will player with whatever the highest number their computer can tolerate is.
What you should do is essentially have no engine limitations that prevent having 100 players (making screens have scrollbars and such). Then set some very high number for singleplayer (32 perhaps), but then have a lot of messages warning the player if they are adding more than the map, their computer, or the game was designed to support. If they want to keep going and have a bad time, that's their choice.
The reason I advocate such a system is firstly because computers will always get more powerful and eventually the lag won't be an issue. Secondly, there will be people who will make mods that would appreciate having a huge number of players (think a deathmatch mod or something like DotA). You don't have to make it that easy to increase the player limit to 100, but it shouldn't be difficult for a modder to do.
How sophisticated can we make dungeons in the game?
Take that one as it comes.
How sophisticated can we make quests in the game?
I wrote quite the post on a system that combines small component tasks and semirandomly puts them together to create a complex and interesting quest. The post is:
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https://forums.elementalgame.com/360530
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How sophisticated can we make tactical battles in the game?
Things that you should include:
-Basic unit facings as they allow intuitive bonuses for attacking from the sides and behind.
-A simple fortify/defensive command.
-Passive special abilities for many units.
-Some extra unit buttons or an area for button popups for units with special innate abilities (i.e. dragons or units enchanted with fire breath actually using their breath attack). These buttons could also be used to expend or activate worn or carried items (caltrops, faery dust, magic amulets). This is key for allowing modders to do a lot of fun stuff.
-Lots of different weapon types with tactically different uses.
Fun, but I could go either way with
-Height bonuses.
-Morale (can easily simulate effects with stat loss instead of making it its own stat).
Things I'd rather not have at all
-Complex unit stances.
If things are to get very complicated and customizable I would rather that part be with unit equipment, training, and enchantments than having too many tactical options in battles.