I don't know of a way to let players have multiple mercenaries, so it won't be in HUv1.7. I'm not yet convinced that multiple mercs is possible, but the limitations I've seen for hardcoded solutions (we can't seem to reduce the %ED buff given to Ethereal items because so far the normal solutions crash the game) make me skeptical. I'd agree that for a single-player game it wouldn't be hard to balance. Consider looking for more experienced D2 modding opinions on
PhrozenKeep.
I'm not familiar with the Sigma mod for D2, but if their online community has decent numbers, then I wouldn't blame them for trying to emphasize the multiplayer aspect of the game. If you're part of their community, then let one of their modders know about this post explaining an easier method to generate a Solo edition for a D2 mod:
http://forum.arimyth.com/viewtopic.php?f=25&t=6644For adding additional types of mercenaries, I believe that can be done, but I haven't done any experimenting myself. The hirelings.txt file has an area where you can assign something from monstats.txt as a hireling. With that in mind, I would expect it to be possible to copy-paste the row for some demon to a row named DemonMerc, and then create a row in hirelings.txt for DemonMerc. You'd want to choose a monster that you know has some animations for attacking, getting hit, spell casting, dying, etc (I'm not sure what the minimum animations for a Merc are).
I don't know if it's possible to give the "Hire Mercenary" option to an A4 vendor, but hirelings.txt make it look very easy to add a DemonMerc to one of the other Mercenary vendors. In some HU versions, new mercenaries were created, but they had the same look as other mercenaries. In HUv1.7 there are A5 Barbarian Mercenaries that shapeshift into WereWolf (survives via life leach and Regen) or WereBear (survives via Stun effects and Regen) forms. Their shapeshift skills are not the same as a Druid's due to limitations in hireling AI, but they have the same stats.
It shouldn't be hard to create a new boss fight that involved any critters you wanted. One harsh limitation is the animations that exist for the more interesting monsters like Tyreal (in battle he can only stand still with slightly wavy wingy things. I suppose he could teleport like Thrakk (no walking animation for Thrakk)).