It really depends on if you're playing Vanilla Skyrim or if you're using one of the few very popular Skyrim Overhaul mods (Skyrim Redone [which is what I use], Perkus Maximus, and Ordinator). However, the whole point of a demonologist/warlock is that you stay at the back line while your summoned minions do the frontline fighting for you. My suggestion would be to specialize in fire damage, but the other two elements are perfectly okay options as well.
You'll want to get the master perks in both Destruction and Conjuration, regardless of which version you choose to play with. This will make the cost of all spells way more manageable. Before trying for the Sanguine Rose, get your hands/paws on a Summon Flame Atronach spell. That fiery beauty is a really good early summon that can save your hide from time to time.
A perk that is an absolute must for Conjuration is Twin Souls, for both Vanilla and for Skyrim Redone (not sure if that is featured in the other two listed overhauls but I'm going to assume that they are). The more summoned minions you have on the field to take the brunt of damage the better.
Now, Sanguine Rose is a really good summoning staff if you don't know how to summon Dremora, but soon as you have the powerful Summon Dremora Lord then you may as well stick the Rose onto a weapon display or inside of a case as it is now no more than a collector's item. However, if you're using SkyRe then summoning any non-Rose dremora comes at a great cost: either a human heart or most of your health.
Good secondary support trees are the schools of Alteratio, Restoration and Illusion. Alteration gives you that extra defence that you so desperately need, Restoration is for when you get flanked or they break past your otherworldly meatshields, and Illusion is good if you simply want your minions to survive the fight long enough by forcing a foe to flee, fight his/her fellows, or simply removing him/her from the fight through paralysis.
My personal recommendation if you're playing a Demonologist in Skyrim Redone: Path of Flames. You can diversify, sure, but fire is much cheaper to use, and your Flame Atronaches get boosts.
Been experimenting with Magic-based builds for a while, with my most successful one being a Lightning based Artificer (made so many dwemer automitae that I basically had my own portable legion with me at all times. Lagged my computer so hard. XD) It's really fun to just panic-cast your way out of situations in the early game, and then become so powerful in the late game that hardly anything can touch you so long as your aiming is spot on.