I would've written this in massive paragraphs, but honestly, that'd be an egregious amount of time wasted, so instead here's a summary of what I personally think the mod can improve on / introduce in future updates:

- Remove the blindness aspect of sunlight exposure or make it apply after a certain healthpull is reached (<50%), IMO it's mostly annoying and not challenging. Vampirism would be a lot better if the drawbacks of it came in the form of regular buffs like weakness, and obviously the sunlight damage.

- Scale sunlight damage with player progression. E.g after entering hardmode or beating a certain boss. It makes the day a consistent threat for the player to deal with throughout the game.

- Add more methods of avoiding sunlight. E.g. Suncream; could be sold by an npc for a hefty price and grants sunlight immunity for 5-10 minutes? Also make the umbrella grant full immunity to sunlight burn; I understand that it threatens to replace blood vials (which I presume is the reason you choose not to add this feature), but again, if you'd ask me the umbrella is already a hindrance on its own for the player. It's pretty much useless after defeating any pre-hardmode boss and it'll only serve to traverse through the surface on daylight, which even then puts the player at risk of fighting surface enemies and forced to switch out of the umbrella and endure the sunlight burn (consider the sunlight scale concept, it can be made deadly)

- Tweak blood vials and make them better for healing + sun immunity. E.g. make them like healing potions for vampiric characters; have lesser, medium, greater, etc, each heal for about 50-70% less of a regular healing potion but also give the hyper regen and sunlight immunity. (Make them also damage non-vampiric players when drunk and/or have them contract Porphyric Hemophilia.)

- Nerf regular healing potions for vampiric characters: make them heal half of their original count, double the healing cooldown, or even make it harm the player. Either way, it'll promotes blood vial usage more.

- Craft gold critters into golden blood vials: basically golden delight but for vampiric characters. It'll feel much more rewarding than just crafting them into five basic blood vials.

- Develop a perk tree-style power system for vampiric characters and introduce vampiric weapons: this one's the most blatant update, it's already been mentioned by someone else in the comments, and you've already considered it, which is great! Life-leeching bites, shapeshifting into a bat (like Vampire's catalyst from thorium) or a wolf, gaining more passive buffs like dangersense or hunter- the whole shtick. Obviously leave this for way later when you've developed the right skills and knowledge to make something like this, since you mentioned you're perhaps learning to create UI, there's no rush.

That's pretty much everything I've come up with thus far. Of course, this is just my opinion and you can do whatever you want with these suggestions. Regardless of whether or not you implement these features, i'll still gladly keep track of this mod's updates.