The main problem I would see is timescale.
The present plan, possible with the present partners, is to have the first engineering airframe airborne for 2035. Add Germany to the mix and negotiating work would add at least a decade to that - which is not available - plus the cost escalation would equal if not exceed any contribution.
Then there is the design. The UK and Japan need a twin engined long range interceptor for long range oceanic type patrols and intercepts, Germany needs a having bought F-35, needs a short range dogfighter (which is what Typhoon was designed for). I can’t see the present partners making that compromise.