I'd go with either Time or Storm, as each are pretty powerful sets. Storm is fun for reasons you probably already understand. It's a lot of chaos and destruction that results in a very solid control mechanism which just so happens to put out a lot of damage. There's something fun about having all your pets, lightning storms, and Tornado's munching the enemies into dust.
Time is pretty good on masterminds, I would think. I compare it too a more aggressive version of Empathy. You have some heals and some team buffs, but you also have a lot of potent enemy debuffs and control moves. The end result is a very capable mastermind who can both keep his team alive while simultaneously deteriorating the opposition.
As for wanting to be more active with Demon Summoning, it's actually not a terrible waste of time to take your whip attacks as they all lower the enemies resistance which would allow your demons to more quickly dispatch opposition. The duration of the -res effect is kind of low, however.
EDIT: Quickly crunching some numbers, I think an attack chain of Crack Whip - Corruption - Lash - Corruption - (Repeat) at about 200% recharge would work pretty well. You'd be able to sustain 3 stacks of -res procs assuming you do nothing but attack for a total resistance debuff of about 28.14%, which is 28.14% *more* damage all your pets would be doing against a single opponent if you have them group up on one target. Could be useful for quickly taking down a boss, or something. Only problem is that you can't really sustain that much recharge as you level up. I can't think of a sustainable attack chain for only 95% recharge, which is what you'd more realistically have on the way to 50. I *suppose* you could just spam all your attacks whenever they're up, but you'll still have gaps in your attack chain, which you probably will need to buff and stuff anyway, and you'd likely only be able to sustain one or two -res procs, which will still help, but it's obviously not as good as three.
Nothing other than gut feeling but Im having a blast with my demon/time....I fell like a troller and my minions are my team !!!
Time is definitely something for those who like to be busy. (geddit?) I also recommend traps.
And i apologise for my severe case of bad pun :P
I tried making a Demons/Sonic and get bored with it, because Sonic is very set-and-forget. I haven't tried slotting the whip attacks much, but I'm not sure they are worth it, even though it'd give me something to do in addition to bossing minions around.
I'm looking for something more active. I have a Bots/Trap and have too many Dark Miasma toons already, so I'd like to avoid those secondaries.
I rolled aDS /TA toon this morning, but I see lots of bad comments on this combo in the forums, and I definitely like my toons to be on the uber side, so I'm reconsidering.
There are the healing secondaries thermal and pain, but I tend to be more of an offensive player. I didn't enjoy playing a DS/thermal during beta, although it might grow on me.
This leaves Time, Storm and Poison, is one of those especially good. I've got a chaotic Fire/Storm controller I enjoy. I tried poison years ago (i12?) and it was OK but it may have changed since then.
Not sure how Time works for an MM.
thw toon will be witch oriented for theme, I want to take advantage of some of the costume pieces (bats and pumpkin head especially) so time is limited.