In 5e, the spear is a simple weapon. (Literally.) There's not much to recommend it over martial weapons if you have access to them.
This is a problem we can fix for the spear-lovers out there by creating a spear for all seasons, like so:
|Battlespear (1)||10 gp||1d8 piercing||3 lb.||Versatile (1d10)|
|Greatspear (2)||30 gp||1d12 piercing||6 lb.||Heavy, two-handed|
|Longspear (3)||20 gp||1d10 piercing||6 lb.||Heavy, reach, two-handed|
|Quickspear (4)||25 gp.||1d8 piercing||3 lb.||Finesse|
(1) - It's a longsword.
(2) - It's a greataxe. Could also be a 2d6 damage weapon, as per the greatsword. Whatever you like.
(3) - It's a glaive. Actually, it's already in the game as a pike but people get scared off by pikes when they have a spear fetish. Don't ask me why. But hey, maybe renaming it a "longspear" will convince your DM to let you use it with the Polearm Master feat. I'd allow it.
(4) - It's a rapier. Now your swashbuckling rogue can be that spear guy from Game of Thrones who got his face crushed.
This is a method that can get you where you want to go. And nothing will be broken because you're already using the tried-and-tested stats for weapons that already exist in the game. No need to add new properties, complex rule kludges, etc.
Need a bunch of Castlevania-style chain whips? Change the damage type to bludgeoning.
Think the idea of dual wielding two rapiers is goofy? (It is.) Write down "Parrying blade (rapier)" on your sheet and use the stats for the rapier anyway.
Want a bludgeon with the finesse and/or light properties so you can play a thuggish rogue who sneak attacks with a blackjack? Shouldn't be hard to figure out, amigo.