I'd love a modern G3-SG1. There is a G36 'pro-line' recoil, but it's TM so would take whacking a metal receiver in there. Doesn't get much publicity out the recoil series, everyone just goes sopmod or scar
I'm currently working on my own mutant SL8/36K hybrid, similar been done by others many times, so nothing unique.
My current favorite bit o HK is my TM MP7 AEP. Has a new Laylax front grip, TM Pro-Sight and Tracer. Gearbox has a fair bit of Laylax in it too, but not much you can do with it due to no available aftermarket motors or gears