If I were to do a suite of DDI tools, here's what I would offer (I'll get to tiers in a moment):
Character Builder: This is
the app that will shift subscriptions. Frankly, 4e is now sufficiently complex that I now won't consider playing it without.
Rules Compendium: Likewise, we know what this is.
Virtual Tabletop: This is the other of the 'big three' tools for a DDI.
Dragon magazine: This should be a very solid magazine released on a fixed date every month. It should be heavily focussed on the player side of the fence, and include new powers, magic items, feats, and so on. Every month, there should be
something for every class and/or race.
(There should probably be a number of 'fluff' pieces as well, such as the editorial, letters to the developers (providing a guaranteed response from the team on at least some topic every month), Confessions, advice articles, articles fleshing out some aspects of the setting, or whatever. But the meat of the magazine would be the PC-centric articles.)
Dungeon magazine: Likewise, a very solid magazine released on a fixed date every month. Ideally, it should have at least three adventures every month (one for each tier). Additionally, the magazine should try to have a mix of adventure types: short zero-prep delves for the time-strapped DM, larger standalone adventures to drop into a campaign, and an ongoing Adventure Path.
(Again, it wouldn't be bad to include some other materials, perhaps fleshing out the setting, or developing an entirely new online-exclusive setting, or giving advice on building a campaign, or whatever. But the meat of the magazine is always going to be the adventures.)
Incidentally, about the fixed release date and the Adventure Paths: these are to build buzz. If the magazines are solid products (which they had better be, or they can be dropped), then we want people to be anticipating them. What's happening next in the Adventure Path? What's next for my character? Releasing the magazines all at once makes this easier - it's really hard to get excitied for "a single article, released when Wizards can be bothered". (I know that every month I used to look forward to the day the magazines dropped through my letter box; I still look forward to the arrival of Pathfinder... despite
never actually having played any of the adventures therein.)
Adventure Builder: This maintains a bank of the
existing D&D tiles (maps, whatever), the
existing D&D traps and hazards, and the
existing D&D monsters (and any 'encounter elements' I've forgotten), and allows the DM to drag and drop these into an "adventure file" to create their own adventures quickly. The key point, though: it doesn't itself allow the entry of custom elements.
Monster Builder: Allows the design of custom monsters.
Map Builder: Allows the design of custom maps.
Trap Builder: Go on, guess.
(Actually, while we're at it, let's also have race, class, power, feat and magic item builders, too!)
The Archive: This is an expanded bank of
all the custom monsters, traps and maps that everyone on the DDI has entered (and allowed to be shared). In other words, it gives you access to the collected work of thousands of other subscribers.
Character Visualiser: We know what this is.
And I think that's about it.
So, the tiers for access, then:
Copper Tier
For Players: Character Builder, Rules Compendium
For DMs: Adventure Builder
Gold Tier
For Players: Dragon Magazine
For DMs: The 'builder' tools, Dungeon magazine
For everyone: Virtual Tabletop
Platinum Tier
For Players: Character Visualiser
For DMs: Access to The Archive
I think that's the combination of features I would go for, and also the tiers. It means that the basic subscription is a very solid offering that people will want, but it also means that there's plenty of value-add in the upper levels of subscription.
But an absolutely key factor in this is that all of these tools and features
must be quality offerings, and
must be supported with every bit as much commitment as is shown to the physical products (if not moreso). Things have been rather shambolic on the DDI front for a while; unless this gets brought under control then the above is an impossibility.