Alright then, I'm not up on errata, no builder. Valorous Smite is good, gives you a nice burst mark. Sanction will be far stronger than Challenge. I'm inclined to look to MC feats if you can't get enough from Paladin powers.

Fighter
Student of the Sword: Gives you a +1 to hit and mark once per encounter.
Battle Awareness: Once an encounter, MBA someone that shifts or doesn't attack you.

Warden
Defender of the Wild: Once per encounter mark everyone next to you.

Battlemind
Demanding Talent: You get Battlemind's Demand once per encounter.

Swordmage
Arcane Aegis: You get an Aegis once per encounter.

Berserker
Battle Berserker: Daily use of the Berserker Fury feature.

If you are a Half-Elf, you may be able to dip into something that way. But to be a solid defender, you are going to have to give up some of your striker potential. You still do a ton of damage and have solid defenses that will help. Make sure you have enough health and surges to handle being a primary target.