Starts in Season 4.
Open Bidding System for the International Managers job using RP this bidding system is run the previous season.
Once the season starts International Managers have a new squad called the country, e.g. 'Brazil National Team'. They can pull players in and out of this squad as they see fit. This works like a shortlist the players don't actually move.
There is an International Cup, which runs regional qualifying of groups of six teams top one or two (depends on numbers) go through to finals of 32.
Three times during a season there are 'International Days' where 3 or 4 of these qualifying games are played. Players at midnight arrive in the squad on a 0% 24 hour loan, (Loans shouldn't count towards rep in Biography anyway?). They will only be eligible for the new game classification 'International'.
International Games can result in injuries but never cure players. Injured players can't be called up.
The Qualifiers are played in quickplay format where the home manager sets the time, could actually do this using home playing rights.
The finals are held in the close season, all International Players move to their International team during this time on a 5 day 0% loan.
Targets and Achievements - Managers love the Biography page, something like that on the managers profile, International Wins - Group & Finals stage. e.g. As an International manager Joe Bloggs has a proven record taking the unfancied Jordan to the finals in Season 14 and taking Italy all the way in Season 6.
Other Notes
Managers can opt out of having their players called up but their players will be limited to Very Good Morale (no superb).
The number of grads coming from a country is made more transparent and directly correlates to performance in the International Cup. This is crude concept numbers but say Italy normally produces 100 normal grads a season if they win the World Cup then bumper harvest following season of maybe 150. If they don't qualify then it's down to 50 if they got knocked out at group stage then it's 75.
Heck you could even make an entire sub game where you seed teams and award RP on the target. So say San Marino were lowest seed you'd have 1m RP for finishing 4th in their group where as Brazil you'd get 1m RP for winning the International Cup.