If it's a clone, then you DON'T want the Fighter to have Warlord options. A clone should try to be as faithful to the original system as the licensing and copyright law allows it to be. If you start giving Fighters Warlord options you defeat the whole purpose of making a clone because you're making a new game instead. To that end, the rationale for my choices would be similar to what Jacob Lewis used for his list, except I'd ignore worrying about covering all of the power sources and think instead about just each role and what I feel is "iconic" about 4e class choices. Additionally since the original question is asking which classes to tackle first, I think that complexity of the mechanics behind them should be considered too. So my choices would start with:Fighter (Defender)Cleric (Leader)Rogue (Striker)Wizard (Controller)The core 4 have to be there to be a clone of almost any edition of D&D IMO. If you don't have these 4 represented, even if under different names, it's hard to consider your engine a D&D clone. The next 3 that I'd add would be:Warlord (Leader) - a 4e clone without a Warlord would not feel like 4e to me. The Warlord is an innovative class that makes 4e very distinct from other editions of D&DPaladin (Defender) - Having a second Defender in your base rules is a good idea, and the Paladin is the one that feels right to me for the base gameWarlock (Striker) - I like Warlocks, so sue me
But 4e was really where the Warlock class came into its own, and I personally think a 4e without a Warlock in it wouldn't feel "right"The last class is the hardest for me - arguably another controller would be the "correct" answer and the Druid would be the "right" one for ties to previous edition reasons. But I personally think the 4e Druid class had a bit of a lack of focus to it and arguably it's going to be more complex to tackle a clone of it and it might be better to hold off on it for later. I feel like Ranger might be the actual "correct" choice even though it's another Striker role - I think the mechanics of a Ranger would be an easier lift than the 4e Druid for a first pass.