Here are some of my ideas.
1) Daniel Boone (Indian Advisor). When Boone joins your congress, at least one Indian tribe will definitely join your cause when you declare revolution.
2) Casimir Pulaski (Military Advisor). When Pulaski joins your congress, all mounted units gain a 25% defense bonus when defending a colony.
3) Ethan Allen (Military Advisor). When Allen joins your congress, Veteran Soldiers are promoted to Continental Infantry or Cavalry faster.
4) Jonathan Trumbull (Political Advisor). When Trumbull joins your congress, Elder Statesmen produce Liberty Bells 25% faster.
5) Nathan Hale (Exploration Advisor). When Hale joins your congress, your Scouts can be used to spy on foreign colonies.
What do you guys think?

I particularly like the bonus you assigned to Daniel Boone, since it is so different from the rest of the Founding Fathers. I have also learned about some historical figures I had never heard about!

My turn:

Blas de Lezo (Military Advisor), defender of the Spanish colony of Cartagena de Indias against the British. He could give a bonus to naval combat in coast tiles, and/or improve the attack of forts bombarding enemy ships.

Catalina de Erauso (Military Advisor), also known as "the Nun Lieutenant". She was a nun who ran away from her monastery and started a military career in South America while posing as a man. She could give a combat bonus in rough terrain.

Francisco de Orellana (Exploration Advisor), explorer of the Amazon River. His bonus could be a reduction of terrain movement costs.

I agree on Nathan Hale, that was my first thought on reading your tagline.

Admiral Nelson - Increase the "stop" range and chance of your war ships making blockades actually effective. Or at least increase the chance if both proved to powerful.

Andrew Jackson - Allow the construction of field fortifications granting a small defensive bonus to a tile.

Benedict Arnold - Convert a unit or tile to your cause. Or some small number depending on how strong it played out.

First I've heard of those three, honestly. Those perks sound about right, though I would say Blaz de Lezo's perk should be the bonus to attack for forts bombarding enemy ships.

Here are a few more I thought of.
Molly Pitcher (Military Advisor). When Pitcher joins your Congress, your artillery do not suffer a reduction of attack when they are damaged.

General Rochambeau (Political Advisor). When Rochambeau joins your congress, the first time you are offered military aid by a foreign king after you declare revolution, the units come to you free of charge.

Francisco Pizarro (Exploration Advisor). When Pizarro joins your congress, natives give you larger amounts of currency as gifts.

Roger Williams (Religious Advisor). When Williams joins your congress, Firebrand Preachers produce 25% more crosses.

Caesar Rodney (Exploration Advisor). When Rodney joins your congress, your mounted units ignore penalties for movement due to terrain.

As a play off your Pitcher idea, I have always wanted the ability to repair the cannons. WOuld be fun to see which perk would play out more advantageously.

Not sure if he would count as a founding father and would probably be available really late in the game, But was thinking Johnny "Appleseed" Chapman. When Johnny Applesed joins your congress, your scouts become able to place forests on tiles without forests.

Thats a neat idea. Or maybe provide a remote colony with some food. Or not let something fall below starvation level.

I would say in his case that he would make Farmers produce 25% more food.

I always wanted the ability to terraform to some degree in Colonization. I understand thats not exactly time appropriate to some degree, but the ability to place a canal would be great.

I think terraforming, even if it's just thd ability to plant trees, should be a trade-off. Make it a pioneer action so at least it costs 20 tools.

Canals would maybe make sense if there was a decent remake and then an Industrial era expansion (to add post-independence content). Funnily enough, in the original Americas map it is impossible to have a canal (other than in the small islands); the only suitable location (precisely in Panama) is occupied by hills and therefore you can't settle a colony there.