I have a "fly tying chest" and a rolltop oak desk. The chest was built specifically with flytying in mind - several wide, deep but shallow drawers lined with aromatic cedar. The rolltop wasn't built with fly tying in mind, but has been taken over. When I built the chest, I also had a "lap desk" - It had a raised section to mount my vise, a magnectic strip to hold hooks, dowels to hold spools, and holes drilled to hold tools, bobbins, scissors, etc. I still use it with the roll top - and I take it with me when I travel (if I'm traveling by auto).
So, my one bit of advice, if you're building your desk specifically for fly tying - roll tops are great for keeping cats (and kids) out when you're not tying, and consider aromatic cedar for some of your material drawers.
Do not tell fish stories where the people know you; but particularly, don't tell them where they know the fish. ~ Mark Twain