First upgrade obviously should be 14Mb ST-Ram board. Popular upgrade is ct60/63/60e with rev.6 68060 CPU... Is it best? I guess it depends what you want to do, but it's ultimate upgrade for the Falcon.
8bit retro is very popular now. There's also ZX Spectrum Next, bus as fas a I know it's not mass produced like TheC64... I guess that 16bit projects will follow, maybe first with Amiga since it was more popular platform than ST.
I don't think there's market for home computers anymore (at least like in 80's and 90's). Computer is just tool nowadays; boring device like microwave oven or television. Retro computing is new home computing ?