It seems that the rage in Items Suggestions now is trying to fill the fourth Boots slot in the shops. Let's compare the current Boots and see whether we'll be needing anything new or better.
Basic: Boots of Speed
Nothing special here, just your average +50 MS Boots.
1. Power Treads
+60 MS, +25 AS, +10 to selected attribute [Str/Agi/Int]
Right. So you've got survivability in this item, and some extra AS for last hitting/denying. The MS bonus isn't as good as it was, but still better than nothing. So I'd say this is Boots for squishy Agi carries, since it buffs Str when set to Str, provides AS and MS.
2. Phase Boots
+70 MS, +24 dmg, Phase [active]
Chasing heroes benefit the most from the new Phase Boots, given the massive bonus damage and the active Phase complements heroes needing the last hit to finish off fleeing enemies.
3. Boots of Travel
+95 MS, Teleport [active]
The Boots for pushing heroes, this really helps you farm lanes and push down lanes fast. What else would you need a Teleport ability for, besides quick escapes from potential ganks or helping out with ganks?
Looking at the 3 categories of heroes using the various Boots well, the one group of heroes missing out on footwear would be the casters. Sure, they could use Power Treads, but they don't really need the +AS. They could use Phase Boots, but the raw damage would be wasted, considering that most of their damage is dealt through spells. They could use Travels, but not all of them can farm up the 2200 gold for the recipe, considering that the gold could be better suited to helping the team in ganks - 2200 gold is 100 gold more than the 2100 of an Ultimate Orb, which could be used in Guinsoo.
Conclusion? Casters need some more love. Since the introduction of Pipe and the nerfing/remaking of Phase Boots, casters have lost some of their previous standing. Perhaps with a decent mid-game Boots for casters, they could regain some of their former glory.