Something that I do often is I make duplicates of low file size samples with different qualitative effects such as some have more Decay for a longer pad effects, or short for a tight little pluck with extra compression, that way when I make synth leads it has some character.
I also take multiples of a lead and replace it with new wave forms. Say I have a saw, I add a square or sin wave, or draw my own until I can get clear resonance from it, then analog filters in the track and fade them in and out to create sweeping effects with my synths.
Finally a big time saver for me is to make templates for things you need (I saved rhodes, pianos, saw, square, triangle, sin waves, two drawn waves, fretless / slap bass, kick, snare, snare reverb, clap, hat closed, hat open, percussions, toms then fx.) and knowing where all the keyboard shortcuts are. Opt + Tab is great for navigating windows, getting comfortable with F9-12 with song sections helps tremendously. Saving modulation tabs and your own FX is amazing. Do it often.
For trills and quintuplets, triplets, tuplets I use:
For Velocity, Pan, Delay Automation, I use this:
I also fiddle with keyboard shortcuts like Saving instruments with CTRL S, Loading with CTRL L, things of that nature to make right clicking through menus less tedious.
Chopping samples with the splice tool is great and setting it to keyboard ranges helps tremendously for making vocals in raps or dupsteps super easy with the new beatsynch capabilities.
Sampling and rerendering samples after reversing, time slow / speeding up with fx in Renoise can make a lot of cool audio fx. Play a lot with that. Don’t be afraid to record your own samples with even a crappy microphone. Renoise can do a lot with a little.
Finally one of the final things I do is I try to make all my synths within Renoise after learning how it works in a plugin because it reduces CPU usage and size by so much.
My main plugins outside of Renoise I use are Tal-4 Reverb. If someone can show me a way to make Reverb like Tal-4 using a mix of compressors or something with mpReverb that’d be great.
I also use Camel Crusher for Compression / Distortion. I used to use Sausage Fattener until it’d reset its preferences every time I’d open up Renoise. 
I’m still looking for ways to replicate FM synthesis, or synth qualities to wave shaping without Serum or Massive. If Renoise user have techniques for similar sounds with that, it’d seal the deal for me.
I’ve made about 6 hours worth of game music (250+ songs) in the past 3 years with this process.
Hope this helps and would love to learn more from you guys or make a Youtube video on my process.