You should ask on Spacebattles, not coz it's more robotic but you'll get better recs there. Here expect utter trash CN fics or a bump coz people won't know what you want. I've got my list of best fanfics, a lot of them are regular xovers, a couple of mega crossovers but sci-fi not anime. https://www.fanfiction.net/u/9071421/
Mega cross over There are lots doujinshi related to SRW series (sorry, I saw it once when visiting one of my friend but he didn't said where he got them) Kinda like when you play SRW (Super Robot Wars) where the story meshed up lots of mechas & characters from different stories/anime/manga Ie: - in SRW J you'll find Kira (Gundam SEED) fighting side by side and/or interact w/ others such as Kabuto Kouji (Mazinger), Akito Tenkawa (Kidou Senkan Nadesico) and/or Sousuke Sagara (FMP) - one doujin I saw pictured Boss Borot (Mazinger) fighting side by side w/ Xabungle - another doujin shows Gundam ZZ in mock battle w/ Mazinger where it unleash it's very own ZZ Punch (seems like it's the inspiration for Gundam Tryon of GBF, LOL)