"Silverhand's Nostalgia Awakening"
Everyone gets one shot. 20 levels and then 20 levels after promoting once (Donny would be allowed to second seal once. No master seals for donny. His stats as a promoted unit after 70 levels with aptitude would trivialize the game).
You only get ONE brave weapon of each type, At most 2-3 Rescue staves and you're only allowed to shop on the map you just completed. Once you miss a shop its contents are no longer available. To make sure you can at least complete the game you get to go and buy Bronze Weapons, Fire tomes and Heal staves whenever you want. Nothing more.
No pair up and supports are under the old 5 point system. One A and a B or any variations thereof, (Chrom and his wife get an extra point because of the forced marriage)
and that means no children bar Lucina and the child of whoever Chrom marries.
No DLC/Spotpass/Renown of course as that didnt exist in older FE's
Play it on the highest difficulty you can feasibly do but it needs to be Classic.