Well, Slayers are fighters too. Is there a reason he needs to be a Defender at all? Or is it an aversion to Essentials classes?
Slayers are fighters by flavor, but they are
not defenders. They can't mark or equivalent and they have very little control (they can start knocking things over at 7th-level, but that's about it).
I'm not saying they suck. They dish out a lot of DPS. They're just not defenders.
Quick backstory: I usually play a warlord/warlock, but the group's pure fighter tank has been missing from games for 6 weeks running. Last week I agreed with the remaining two other players that I'd rebuild as warlord/swordmage to take up the tanking role, but then out of the blue our tank turns up again to last night's game. We ran into a problem instantly with us both needing to mark targets to activate our tanking powers, and it was especially problematic against a single powerful target. It was good to have him playing again, but given his current unreliability at games, we suggested that maybe he should rebuild into a dps role rather than an essential tank role. We just dinged level 20 last night by the way, so I'm very close to the swordmage total aegis power.
I'm no expert on 4e (grew up on 2nd edition ad&d) and I'm wondering:
a) can fighters even build as dps to compete with a striker class?
No, but you can deal more than respectable damage if you use a greataxe/maul/greatsword and focus on encounter and daily powers that simply add dice without special effects. (For instance, Brute Strike is a 1st-level daily power that lets you 6d6 + bonuses, and I think it's reliable too.)
b) can fighters provide good dps without marking targets?
Yes, but marking helps a bit. Marking targets gives you extra attacks if someone shifts or otherwise violates the mark, but rarely gives you bonus damage by itself. Of course, these are basic attacks, so any item that gives a flat damage bonus to melee basic attacks can't hurt.
Note that Combat Challenge applies any time you make an attack roll. This includes a dragonborn's breath weapon attack (close blast 3 or greater with feats) as a minor action. This can be huge, although it won't do anything about your reach.
And of course, Potent Challenge gives you bonus damage whenever Combat Challenge gives you an extra attack.
c) any particular advice for a dps fighter?
Yes. It seems the answer is in feats rather than powers. There's a few fighter-only feats to boost damage (Weapon Focus) and depending on your weapon, many powers deal bonus damage. Hammers and axes tend to get the greatest benefits out of this, so scrutinize those powers. (Mauls are probably the best for damage dealing, although they only give a +2 bonus to hit.)
Dwarven Weapon Training gives a +2 bonus to damage rolls with axes and hammers. Some other races (eg eladrin) gain similar bonuses, but eladrin aren't really a heavy damage-dealing race.
Also, Hammer Rhythm. Deal damage even on a miss. Reaping Strike is also good for this, but the benefits do not stack, whereas Hammer Rhythm works on missed encounter and even daily powers.
Powerful Charge gives you a bonus to hit and damage while charging, and it's an untyped bonus. If you're willing to sack a feat on Mounted Combat, a horse gives +3 damage while charging, but the combat value of a horse falls quickly with levels, and no other mount seems to have that ability, so probably not the best idea. (If you can find some sort of "paragon/epic horse" though, go for it.)
Another not-so-great idea is to get an elemental weapon, which lets you do elemental damage every turn (so, gives you that elemental keyword) and then take a feat that gives you bonus damage whenever you use such an attack. It's not just for spellcasters anymore. On the other hand, if you took a feat that gave you bonus thunder damage, and you later switch your thundering sword for a vorpal weapon, the feat promptly loses its value. (Falls under the "probably bad idea" like Mounted Combat, in other words.)
Dragonborn Frenzy gives you +2 damage while bloodied, and dragonborn also gain a small bonus to hit while bloodied.
Power Attack can give you +6 damage with a two-handed weapon at paragon level and +9 damage at epic level. Worth it? It applies to any melee attack, not just melee basic attacks.
In addition, fighters are pretty good at getting situational attack bonuses. Blade Opportunist and Combat Reflexes give bonuses to hit with opportunity attacks (they don't stack), and hitting more means more damage. (If you compare a rogue or thief to a barbarian, the former have higher accuracy than the latter.)
d) any other clever solution to us both needing to mark targets?
Well, the Essentials Knight practically has a "marking aura" around itself. The Knight also has a couple of at-wills that boost damage (one with a penalty to hit).
