How about getting G2 and using it for sequencing such stuff? You can create sequencers of your wildest dreams
BTW not having this feature in MD is not matter of hardware limitation contrary to what Futureimage said. It's purely software issue. But I can see how this would complicate everything in the MD... what would happen if you change pattern while some of the tracks would be yet to be finished? I don't think Elektron is never going to implement such feature - it's a great feature on its own I guess and some people would certainly use it but it just makes developing sequencer software A LOT more complicated. So it's not probably worth it.
For Hageir: Arpeggiator is a special sequecer which generates note events to the actual sequencer. Think it as a sequencer inside sequencer. But it doesn't help with the track length issue I'm affraid