7First try, please don't flame
Disclaimer: I own nothing other than this idea
Ddraig and Albion were slain that day they battled against the three fractions. However due to their symbolism of Wales and England, they ascend to the throne of heroes. A thousand years later, the red dragon is summoned by Issei Hyoudou under the Beast class.
Must have:
Change Issei's power pool from almost nothing to extremely high even for an heir of a high magus clan (doesn't need to be a one.) Ddraig needs a lot of Prana to stay maintained, never mind using his Noble Phantasm.
Ddraig must retain his original (Kyuubi-like) personality. He attacked the three fractions and their leaders (including GOD) because they interfered with his fight with Albion. Here he was not sealed in the boosted gear; therefore he hadn't got a chance to mellow out. Therefore he's unlikely to obey any order that isn't enforced with a command seal. He only accepts the contract at first because Issei is the highest Prana battery he can find at the moment and he won't lower himself by eating human souls. (He may warm up to Issei eventually,( up to you.)
Keep Issei at least in somewhat character. Change his dream, make him more intelligent give him better self-control BUT DON'T MAKE HIM ISSEI ONLY IN NAME. He's Iskander on sugar rush, not a less prideful Gilgamesh.
Don't make every battle with Ddraig a curb-stomp. It quickly becomes boring otherwise. Reasons for why not may include: Prana shortage, the opponent is a dragon slayer or Issei been in the way.
Make sure the servants curb stomp anyone who isn't an experienced fighter.(more Prana and less Gaia resistance remember.) If they are not stronger than most, then at least they are more skilled and deadly than the average sacred gear user. The power of destruction isn't going to be much use if the enemy keeps dodging it.
The Holy Grail War could start before or during the start of canon. If it finishes before Canon, Issei is probably going to suffer of shell shock.
Regardless of starting and finishing point, at least some people from different fractions are going to try and win the grail. Just because the leaders (and anyone with common sense) want peace, doesn't mean everyone does.
NO CRACK OR BASHING!
Optional
Issei being human when he summons Ddraig. However if you chose this, Ddraig is probably going be against peerage. He wouldn't be downgraded into a glorified familiar of a servant of a spoiled brat.
If Issei summons Ddraig in a multiple servant war like apocrypha or grand order or a multiplied normal war, you may include a Vali and Albion/Beast team. If they are on the same side, sparks will fly. (At least between Ddraig and Albion, in canon Issei and Vali had no real reason to fight other being the dragons's hosts and being on opposite sides.) If they have different alliances, well, you know what will happen.
Simile Manaka Sajyou (from fate prototype) and 666 may be included in the war. (I have come to realise that Manaka could be used as a foil for Issei. They both were innocent in their own way, both were set on their dreams and both would go great lengths for the person they loved/lusted after. However Issei would never sacrifice anyone while Mankaka was too happy to use her own sister to revive 666.)
If Issei takes part in a war similar to kaleid liner prisma, he probably is going to be affected by Ddraig's memories. To what extent, you decide.
Please the rules in the must have. Otherwise, go wild! Genderbend Issei or Ddraig (why does everyone do it to Kyuubi and not him?), make Issei a magus or angel, the war happens somewhere else other Kuoh or Fuyuki, I don't care otherwise.