Reaktor, Max, etc are good for these things. I did a few songs based on a Reaktor ensemble back in ver2.3, not sure what it was called now, but had a 32-step random pattern generator (that's probably close to what it was called) that created a new pattern with each button press. I couldn't believe how every pattern sounded perfectly usable....that was my introduction to note quantizing. Big fan ever since, especially as I never learned to play scales.
Something like Reason or Numerology could be fun for creating variations, although last time I used Reason the step sequencer did not have a random function. But you could use several to control different settings in a rack synth, set them all to different lengths so you rarely get the same progression of changes.
For a midi loop in a DAW program, there are usually midi effects that can be applied. Say you have a randomizing effect like in Ableton. You could have more control by cutting some notes (like the top third of the pattern) and pasting them on another track. Apply the randomizer to just that too third track.
For modular....well, I can't recommend a quantizer enough. I have three dual quantizers in my system. Stick anything through one, a little offset and trim, and you have instant variable melody. For the clumsy, sausage-fingered, or untrained player, these can take all the worry out of playing a keyboard, and they make dialing in usable step sequences in a live setting quick and easy. Controlling the transpose input can bring variation to a looping, quantized pattern. I sometimes use a variable pulsewidth LFO to create a quick shift, and as the LFO is not clocked with the sequence, I often get that shift occurring during a note's duration, giving an extra little off-time blip (or bend if using glide).
The Doepfer A-172 Min/Max is good for combining signals and extracting two not-easily predicted control signals.
Step sequencers are essential (for me, anyway). I have two MAQs and the A-154/155 combination. The MAQs are cool as they can control analog and midi gear. All can run in random mode (requires the A-154). With the MAQ, I can send row 1 to notes and use row 2 as a transposer for row 1. I can keep most of the steps of row 2 at the base note and just have maybe 4-6 other values on some of the steps. Then row 2 can be set to random so the transposing occurs at random times and at variable degrees. Timing division of row 2 can also be adjusted so that the transpose duration lasts 1/4, 1/8, 1/16th. Always bugs me that I can't divide down to a whole note per step.
The A-155 has a few tricks. It has built-in S&H so notes can be held, just need to use an external gate of some kind to trigger the hold. That variable pulse LFO works well for that, or a separate sequencer. It has the same control over Glide on/off. More importantly and unique is that each step has an input for injecting external signals into the sequence. Quantize the output and run an LFO or two into some inputs and for the duration of those steps you'll get a burst of random notes depending on the LFO shape and rate. This feature should not be overlooked or underestimated. When coupled with the A-154 Sequencer Controller and some LFOs, you can modulate the first and/or last step, run direction, etc.
Similar changes can be made to the MAQ sequences using MIDI commands. This is less "random" as you are making those changes deliberately, but it results in an unpredictable sequence for the listener, and can be recorded as MIDI note data for further editing.
If you have multiple sequencers, it can be interesting to use one for pitch and one for trigger. The trigger pattern can be a loop with changing pitches each cycle, or the other way. Also, you can use the trigger pattern of one sequencer to advance the other so you cycle through all the notes, but not necessarily repeating the same bar often. Sometimes I use one CV pattern to generate two sequences by having different trigger patterns for each voice. Say a 16th note pitch and gate pattern goes to a high range voice with a random direction. The same CV but a much slower trigger pattern (requires a S&H module) is sent to a sub oscillator which will always be in tune with the faster pattern. I've done this in Reaktor and with my modular.
Creating octave shifts can be good too. This might be done with constant voltages and a sequential switch, transpose in on a quantizer, etc, but I also have one VCO that has a CV input for precision -/+ 1 Oct. I can use a joystick, LFO, or whatever to shift the pattern up or down.
Using a clock divider/multiplier is also good for creating "ratcheting" moments in your sequence. Doepfer just released a dedicated module for this.
Ok, my thumbs are tired