I'll assume that successful means profitable. I'll take 5th edition. It's solid.
So, five products:
1) PHB- all the rules, worded clear and concise, with logical layout. PDF bookmarked, with index linked to precise page. Also, include clear rules for making custom classes, races and backgrounds.
2) DMG - Advice on how to design things ( settings, encounters, puzzles, NPCs, plot hooks). Advice how to run games with examples, how to mediate, how to run good session 0, differences in play styles etc.
3) MM- monsters, lairs, habitats, clear and precise rules on how to build custom monsters
4) Official setting book - all fluff, no crunch. Places, people, world events. Adventure hooks, maybe 4 short adventures ( 1 for each tier).
5) Mobile app/ web based SaaS D&D Companion tool- all encompassing tool useful for both DMs and Players. Character builder, adventure builder, random generator for bunch of things. Subscription based. Enable users to share or sell their home brew stuff (and take reasonable cut of sales, like 30%), allow 3PP to sell their stuff through the app (for cut of sales off course). Sell packs like adventures, premade npcs, new monsters, maps, character art ( new official classes and sub classes are always free). In essence, give good value proposition for both people who play VTT and IRL. People will pay for solid product that makes their gaming more convenient. Advertise it heavily.
And then, i'll cheat a bit with 2 investments.
First, cartoon. Animated series, 16+, so aimed at older teens and adults. 4-8 epsiode, 30 min long, adventure arcs, representing different settings and styles ( from light hearted funny trope filled like Delicious in dungeon, to more serious and dark like Castlevania, horror, high fantasy, low fantasy, sword and sorcery pulp era, you get the gist).
Second- invest in F**ing advertisment. Lean heavily into real life social experience side of it. Lot's of people who don't play ttrpg are playing board games and have game nights. Sell them D&D as fun alternative to try. Show how ttrpgs are good way to meet new people, make new friends. I meet bunch of people through this hobby, some of them became my longest and closest friends, my friend met his wife through D&D. Share stories like that. Present D&D as a fun activity for IRL socializing. Present other benefits, like practicing social skills, imagination and creativity practice, Give good support for VTT, but promote IRL.