Why not include a system where you can choose an era to play in. For example, a Half-Life 2 era has access to all current Lambda Wars maps and factions: Rebels and Combine. Then, if you choose a Half-Life 1 era, then you can choose between factions like HECU or Xen, etc. Don't need to MERGE them, just divide them into two different eras.
I don't like the two base factions, even though they are fun to play at times. It just gets old as you continue to play. Look at Company of Heroes. They can have Eight factions, four for each sides: Allies and Axis. You can fight alongside Soviets, British and American troops in the same match and map and no one whines and complains. They just go with it and it's more interesting to merge all the different tactical and combative styles of each nation/faction when playing with all of them in one match.
It would work better if they were like this:
-Two Eras: Half-Life and Half-Life 2
-Era-specific factions and maps or options in custom servers to allow all factions/maps playable in the same match
You don't have to merge everything if you want more factions. You just have to add them in wisely without messing with the concept of the game, unless that's what you want in a public game on a server, which is fine because it's a lot more fun to experiment and make the experience unique. But, that's on an individual server basis. XD
So, it would be likes this for example, if you were to do this:
-In a multiplayer match, select an era to play in (Black Mesa Era or Combine Occupation Era)
-If Black Mesa Era is selected, make these factions selectable: HECU or Xen (If you were to do these)
-If Combine Occupation Era is selected, make these factions selectable: (Rebels or Combine)
-Make a Mash-Up option available, which allows all 4 Factions to be playable in a match and allows the selection of all maps, regardless of era
-Maps are Era-Specific, but can change after the Mash-Up option is selected by the server host or player if in a single-player skirmish
Now, in terms of developing this idea, why not ask the community if there is anyone with the skills required to make this happen if they are willing to help if you guys are considering this? I think it would be great to see who would step up and be serious while also having some great ideas and draw-ups/models/concepts/etc. to assist in the creation of these new factions. I also think it would be cool to actually ask the community for their help in the development of this mod, as it would help make things easier for you guys and can really be a great help in your development endeavors. You know, there are many talented G-Mod modelers who may want to use their talents in other Half-Life medians other than G-Mod and I think branching out that way could help.
Anyways, I thought I'd just suggest something and add to this discussion.

I am one of the many fans waiting for something like this and would love for more factions to play around with and have fun. I know it's hard to develop such factions, but it would still make the game a much more fun and developed experience that could really lead to some interesting gameplay elements.
