The important part: Only one of each of these items can ever be built per team per game. If they are sold, they're gone. Also, each of these items should build into (at least) 2 legendary-tier items:
1) An item with Tenacity. The only other item that offers Tenacity should be Merc Treads (if even that).
2) An item that is, stat-wise, of slightly above-average gold efficiency, but below-average slot efficiency, and also offers a useful active ability.
(Additional items should also be cost-efficient and have interesting actives.)
Both items should be viable end-game picks, since they're not going to make frequent appearances.
Impact: Supports get their income quicker and can still do their job effectively while doing so thanks to the variety of items offered. Also, early, aggressive play is rewarded, since a kill puts you half-way towards one of these items. Aggression is especially rewarded for supports, who are likely to buy 3-4 of these items. That said, because of the one-per-team restriction, buying multiples of these items when *not* playing Support will almost certainly gimp your support and make late-game effectively a 4v5, or force someone else to share farm with your support and making your team weaker, overall. It's also impractical to buy multiple items on non-supports and be effective because the stats on these items are highly varied. An ADC and an APC really only each want one of these items, while a bruiser or tank might want 2. Supports that need survivability and utility will like collecting them, however, not only because you need a lot of different stats to cover those two roles, but also because the items these build into will improve their utility even further.
First, the existing gp10 items:
Bead of Rejuvenation (250g) + Faerie Charm (180g) + Recipe (170g) = 600g
Dagger (420g) + Recipe (180g) = 600g
+25% Attack Speed
Heart of Gold:
Ruby Crystal (475g) + Recipe (125g) = 600g
Kage's Lucky Pick:
Amplifying Tome (435g) + Recipe (165g) = 600g
Next, existing items that should be gp10's:
Meki Pendant (390g) + Recipe (210g) = 600g
+8 mana/5 aura
Emblem of Valor:
Rejuvenation Bead (250g) + Cloth Armor (300g) + Recipe (50g) = 600g
+8 health/5 aura
Also, ideas for a new gp10's:
Sapphire Crystal (400g) + Recipe (200g) = 600g
Null-Magic Mantle (400g) + Recipe (200g) = 600g
+35 Magic Resist
In addition to listed stats, these items all, obviously, increase gp10 income by 5.
EDIT: Changed the order because nobody is commenting on the important part, making me think they're just not even reading it and looking at these like normal items.