I've tried to look for a way to increase how many units can participate in tactical combat, but I think it could be hardcoded to 9 unfortunately.

1. Yes, we can do that.

2. Yes, we can do that. (if by stack you mean how many members are in a unit, ie 3-5-7-9, those numbers can be increased)

3. Yes, we can do that. But it won't be using the caravan tags, it will be done using the unitlimit stuff for recruits from monster world resources.

4. Maybe. We can create maps of any size, but will the game run it and will the AI function on such a map?

We can't alter tactical maps yet because they've locked the workshop. But they were moddable in E:wom and I would bet my left ball that they will be in FE as well. I'm making the assumption that we can make them larger because a lot of space in tactical maps currently are just background environment. We should be able to build on those tiles.

What I find cool for tactical maps is that from looking at the stinking mud spell we seem to be able to apply particle effects to a tactical tile using a spell and have that tile perform a modifier.

        <GameModifier>
            <ModType>Map</ModType>
            <Attribute>CreateWorldProp</Attribute>
            <StrVal>Stinking_Mud</StrVal>
            <Duration>-1</Duration>
        </GameModifier>

        <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>SetUnitStat</Attribute>
            <StrVal>UnitStat_Moves</StrVal>
            <ApplyToTile>1</ApplyToTile>
            <Duration>1</Duration>
            <Value>1</Value>
        </GameModifier>

The first modifier applies a set particle effect and it looks like we should be able to mod anything we like using the particle editor, ie snow, ice, venom, lightning, etc. Fire already exists.

The second modifier allows you to modify the stats of anyone on the tile. Even if stat modification and damage are the only effects we can apply to tiles it still allows you to do a lot of cool things.

What I'm wondering is if we could apply these directly to tactical maps without having to cast spells. Then we could mod in dynamic tiles like fire on volcanic maps, ice and snow on arctic maps, and mud in swamps.  

Ooh that's an interesting idea. You could add in a Wall of Fire then... I always enjoyed spells like that... Could you also do a "Blaze"( I.e. fire when you've stepped)?

edit: I admit I once was a huge Diablo 2 addict, and Blaze was one of my favorite spells... I had a sorc with a Blaze skill of 35, and stacked it high!