They're pretty much just assassins with pets. Hell you can play them exactly like Assassins. Lord Erebus especially is a very potent fighter, more so than some of the assassin demigods.
Generals are nothing really new or special from the various DotA heros with pets: Undying, Keeper of the Light etc etc...
They need more ways to affect the battle other than wading in and kicking ass. Normaly the player in an RTS is a "General" because he can control ressource gathering, decides what and when to create units, decides where the units go and what they'll do. The current Generals do nothing of the sort other than walk around with a retinue of pets.
- Being able to spawn an extra wave of creeps as a long-ish cooldown ability.
-Possibly being able to control what they wave will be. Being able to get a Giant into the battle a few waves in advance would be amazing.
-Being able to point a wave of creep in a specific direction. Maybe with something like a "rally flag" which they could put down a few times during a fight which would send creeps there instead of to their normal direction. Or maybe this would only work for the General spawned waves of creeps.
-Improve ressource gathering. Possibly by placing some summoned unit on a gold mine they would increase team gold aquisition from mine, and get a little extra for themselves.
-Structure building or structure repair. Another long-ish cooldown ability to repair a mine or fort.
-More buffs. Vampire dude really didn't have that many support buffs and auras.
Hell looking at this list, you could make it so Citadel Upgrades are only available to Generals and not Assassins and you'd really make them feel a lot more like a general. Giving them an extra page in the shop where they could "buy" extra creeps to add to a wave, and an item they could use to repair a destroyed building would be another step in the right direction.
If they makes them too invaluable to have, then at least give Generals a rebait on the Citadel Upgrades, and give them certain upgrades only Generals can get.
There's so many ways you could make Generals feel more like an RTS player, less like an Assassin with pets...