Improving the Effectiveness of Knights in Dungeon Fantasy
Knights are already a very effective template at what they do, but they do have some weaknesses. There are a few tweaks that can be made to the template or to the game rules to make them slightly more effective.
Getting Rid of Connoisseur
Knights have to take Connoisseur (Weapons), even though there's no canonical use for it in DF: Dungeons. Superior weapons are identified by an Armory (Melee Weapons) roll. Replacing Connoisseur (Weapons) with Armory (Melee Weapons) is a simple fix that gives knight a little more utility in identifying and repairing weapons and armor.
Improving Born War Leader
Knights' mandatory Born War Leader Talent covers only 5 skills, 2 of which aren't normally used in Dungeon Fantasy. It's actually not a very good Talent. A simple fix would be to add Intimidation to the list of covered skills, giving knights an Influence skill they can use at some level of skill. Since Intimidation is already useful for forcing foes away (DF2 p 12), this is reasonable and appropriate.
Other skills could reasonably be added to Born War Leader. Adding Armory dilutes the concept of the talent somewhat, but it adds to knights' out of combat utility. Heraldry is a more appropriate addition, and lets knights identify organized foes like bandits and orc tribes. An unconventional option would be to add Kiai to both the talent and the knights' background skill list, letting Born War Leader act as the appropriate background advantage. Using Kiai to represent a powerful battle cry is a canonical option for Medieval European knights, so this isn't as strange as it sounds, and knights tend to be good at Kiai due to their high HT.
Knight! is already an excellent wildcard skill, but there are some changes that can be made to it to better encompass the knight concept. The obvious options are to add some or all of Knights' primary skills to the wildcard skill.
Adding simple unarmed combat skills, like Wrestling, Brawling, and Boxing seems like it would overlap with the Martial Artist, but it doesn't really. Knights are still going to focus on heavy armed and armored combat, though they would have better options for dealing with threats when unarmed. Martial Artists would still be better, with a better base skillset (Judo and Karate) and Trained by a Master giving them improved Rapid Strikes and repeated parries.
Another option would be to let Knight! substitute for all heavy thrown weapon skills, Crossbow, Sling, and Bow. Again, at first glance this is niche invasion into Scout, but I don't think it would work that way in practice. Heroic Archer defines Scouts, and can't be replaced merely by a high level skill. However, adding missile and thrown attacks to Knight! does mean that more people in the delving band can contribute in ranged combat with a reasonable degree of accuracy.
Finally, adding Knife to Knight! saves Knights to save 1 point. Given the general uselessness of knives in almost all dungeon fantasy situations, this is not likely to overlap with Thieves much at all.