1. All maps - probably not going to happen. There are bits and pieces here and there, but the majority of those maps simply haven't been created or weren't included. I found some files referencing a battle between Arthas and Muradin (another tutorial level? or maybe a scrapped tutorial?), but it's very incomplete.
2. Alternate costumes/mounts, choosing bot difficulty, choosing talents, choosing teammates/enemies: This shouldn't be very difficult once the file format is completely reversed because all of this is controlled by the map's MapScript.galaxy file. We first need access to the compiled StormMap files. They are currently compiled on the fly by Blizzard, so that process must be reverse engineered. After that's done, somebody would need to make a small tool to alter parts of the StormMap quickly - shouldn't be hard because the hex offsets will never change since the StormMap is static. Additionally, Ladik's libraries for MPQs also work for SC2Maps (and therefore almost certainly work for StormMaps). This will require time but not an -incredible- amount, so somebody will probably do it eventually.
3. Online/LAN Play: I have no idea. It's going to require the file format to be fully reversed, just like #2, but it will also need much more work done, specifically to the exe. This may be nearly impossible because it would require somebody with a lot of skill and a lot of time to waste (only a small community will use a leaked alpha).
Edit: With some serious tweaking, Heroes could be made to run in SC2 directly as a custom map. I'm doing a little work on it atm. I've recompiled a ton of bits and pieces of one of the maps and recreated it as an SC2Map. It loads, but again, need full reversal of file format to get all the models/textures - right now it's just a blank black map.
https://i.imgur.com/imTFvix.png
All Heroes files are already included - they get downloaded from the FTP and then combined into those massive data.000, data.001, and data.002 files. Accessing alternate costumes/mounts could probably be done with memory editing, but it would take a lot longer to get working than hero selection.