Yuku free message boards

I figure your right that Mounts and Familiars work best this way.

What follows is a mental dump of ideas related to this concept.

Mounts are fairly easy to define, and most of their movement rules are pre-defined in the DMG. An exception here would be that these bonded mounts will let you ride them into anywhere (instead of the most mounts disliking indoors.) I figure this version of Mounts would be a near mystical bond to a special amount. This would allow the mount to have scaling defenses, hitpoints, and attack bonuses to match the PC it's bonded to. I do think it would be smart to provide a base level 1 "mount" set of stats instead of allowing this feat to apply to any of the Mount Monsters that might crop up in the Adventurer's Vault, MM, or future products. You could pick one special feature for the mount... and that feature might provide a keyword (like reptile, spider, etc) as well. Maybe some of those would be feats instead.
Powers, like you said, would consist of Charge, Trample, ride by attacks, re-direct attacks aimed at the mount to the rider, and healing the mount. I figure the powers should all be Martial in nature.
Mount related features would require the Mounted Combat feat plus the a new "Bonded Mount" feat. This feat would not be keyed to any spicific power source (so Paladins, Fighters, and Barbarians could all take it.)

Familiars I would define as needing to be Tiny creatures, and they would definitely be arcane in nature. They would have fewer hitpoints and be less likely to be separate from you very long in combat, and would not be good choices for attack. Because they have no reach, they wouldn't really be able to flank (is that a rule in 4ed?) Base stats would still increase as you level up. Familiars would obey standard rules for companion creatures (requiring you take actions to give them actions) but with the exceptions that your familiar can always occupy your space with you, you can chose to have your Familiar remain with you when you move as long as you start your turn with the familiar in your square, and your Familiar in your square does not take damage from Auras, Bursts, or Blasts. It can still be targeted specifically by attacks, but gains cover from you. Effects other than damage may still effect the familiar.
Base familiar stats would be generic, but you could pick one special feature for your familiar: such as flight, low-light vision, crawling, swimming, tracking, etc. You could describe the creature how ever you'd like (spider or rat are both crawling familiars... or a flying one is a mini drake, whatever). Familiars would somewhat smart (compared to animals) and could communicate with you via a very short range telepathy (that only works for you.) Without taking other power swap feats they would be good for scouting (it should have decent perception and stealth checks) as they could travel away from you, spy, and report back.
Familiar powers attack powers would allow you to shift (or at higher levels teleport) your familiar to a location and then cause a magical attack to emanate from it, magically enlarge your familiar and give it an attack (sustainable), etc. Utility powers would help it with scouting (such letting you see through it's senses and issue it orders over a distance), let you enlarge it into a mount for a while, and other even stranger uses.
The "Bonded Familiar" feat would be an Arcane feat (requiring you have the arcane power source).

I would consider both these "Bonded X" feats to be Multiclassing feats, so as to limit most characters from multiclassing and having mounts and having familiars. I say most characters because Bards would be valid to do so.

For both cases feats could also be provided that let you further customize your familiar or mount. Similar to the Beast customization feats in Martial Power. A mount from the Feywild that teleports whenever you do (good for a Feylock or Eladrin). A skeletal mount or familiar for those necromancer/death knight types, or immortal origins, or living constructs... (maybe a paragon feats for some of those.)

Rituals could be provided that raise a dead Familiar or Mount cheaply. Also, a ritual could be devised that lets you summon or send away your mount or familiar, so you don't have to lug them around ALL the time. instead you could take a quick 1 or 5 minute ritual to summon them when you need them, and a similar one to send them away if they aren't needed. These rituals would be cheap and low level to cast. A character would know both the raising and the summoning rituals if he takes the Bonded X feat.

Adventurer's vault already contains rules for Barding, as well as companion and mount related magic items as well.