I think Brooks is a more direct cover for Duffy, although I also like to see Brooks deployed as a false 10 (damn I used the term) when he comes on for Sharp.
Carruthers for me sits somewhere between Fleck and Duffy in terms of his ability and approach.
I can see Lundstrum as a (albeit not quite so effective) cover for Coutts.
But I can't see a direct cover for Fleck, which to be honest isn't surprising as he's one of those fairly unique players. We could always put a cheeky bid in for Barry fucking Bannan as cover for Fleck. He'd do alright in our U23s until needed
One player we haven't seen much of yet is Thomas. He looks really promising and would love to see him develop into a first teamer, although I can't see where he would play as he's an out and out winger. Maybe he can learn to adapt and play more central as cover for Duffy?
I'd also be looking at more cover for wing back. We've seen how important this position is to our style of play and how vunerable we are to injuries in that position. With the amount of work the WBs have to put in it's inevitible they're going to get knocks and strains. So maybe a WB who can play left or right as cover (if such a thing exists - I played that role as a young man and it's true, I was equally shite left and right).
We also now need cover for Clarke... How about that fella playing in France - Neymar isn't it? He could learn alot as Leon's deputy