Ok, I can see that many people have many ideas about this.
I have to say, the base idea, for the ability to grind out 'Prestige' versions of ships is one I like. I think it has real potential. I have 12 lines which I have completed. That's not a boast, I just like playing a lot. I'm about to finish the Russian BB line too. So, for some lines, having the ability to boost one ship in the line (likely the T10) is an attractive drawcard. I really do like the base concept.
That said, there are a number of issues that would need to be addressed.
1. What happens to the unspent XP on ships? Not everyone has either a premium account or is willing to spend hard earned money to cash in ship XP for Free XP. I know of people with over 3 million XP on some ships, simply because they dont want to spend the money, but rather they just enjoy the ship. They have had the ship for years and love playing it. (Looking at you Yamato). Does this ship XP just get lost? Does it get converted to Free XP? Is it kept in reserve for when you re-grind the ship? If it is lost, is that fair on the player?
EDIT: Just checked, it goes to the T1 ship of the line. Well, i'm not sure if that is worse? I'd have to win Lotto, one of the big payouts, before I'd have the cash to be able to afford to cash it all in. Why don't they just keep it on the ships? Held until yuo have re-ground to the ship and then apply it again? Oh, Wait, that would mean you arent spending cash to turn it into Free XP.
2. We know that the exact details of the promotion/prestige/whatever bonuses will be changed from the initial blue sky examples given. However, it is still going to give a significant bonus to the people that have re-ground the lines. How do you prevent the seal clubbing that will result? When a newish player has his brand spanking new, still smelling like the dockyard first ever T10 and enters a match against the same ship, but is Prestige Level 3, he is at a massive disadvantage in the quality of the ship. Sure, consumables will all be free and Premium now, and that doesnt really matter because the other ship has more HP, faster consumable reload and so on....
3. It wasn't specified if having a line re-ground would be an option or not. Some people may be happy to stick with the vanilla Rank 10 ship, some may want to go through the grind. Will we get a choice? I put many hours into getting my Des Moines. To log on and find it gone would infuriate me.
4. Legendary Modules. Would we have to re-grind those? Just getting them is a huge effort. To suddenly need to re-grind a line and THEN re-grind for the legendary would leave me so frustrated, I may never touch that line again.
5. How many times? I mean, from the descriptions, it varies depending on interpretation from 3 times for maximum bonus, to 7 times that you can re-grind a line.
6. How long is a Season? I understand that you'd only be able to grind 3 lines a season, but how many days is that? Is it ever specified anywhere? Would I have 3 months? 6 months? A Year?
7. Premium Ships. Would we be able to apply the Prestige points to a premium ship? It's one thing to be able to boost your standard line ship, but will we see these bonus' applied to the Stalingrad? The Tirpitz?
8. Credits/Silver. You would 'Sell', your entire fleet and have to re-buy them and all the modules. Without using Dubloons, that means you only get 50% of their Value. Now, to buy a new T10 BB costs 20 Million credits. Not counting the extra, what? 4 Million for Modules? Because you have to re-buy them all? The biggest slow down for some players is the grind for credits. I know players who do 5-10 Operations a day, just to get Credits. If I had to re-grind a line, without all the credits, I'd be tempted to walk away. It's flat out saying to the players that you need to purchase Credits to make any progress in these lines. Because the Credits you earned to buy these ships? Half of them are gone and thats, more than 50 million credits for a BB line.
9. Why make the bonuses the same per class? You have a perfect opportunity to have a range of 'upgrades' that can be purchased which would apply to these prestige points.
Lets take an example. From what I have seen, If I was to grind the Russian DD line and the IJN DD line at the same time, The same bonuses would apply to the Khabarovsk as well as the Shimakaze. Which is silly. Frankly, if I am looking at upgrading my Khaba, I don't care about the Torpedo reload time, the torp damage and the like, I care about speed, rudder shift and my guns. In the same vein, My Shima is primarily about the torps, but I may want to boost the gun power and turn it into a very very nasty Generalist DD. OR boost the Torps even more.
Why not simply have a list of 'Upgrades' that can be purchased?
For example. I could buy from a list of 10% more HP, 10% faster Torp Reload, 10% faster consumable refresh, 5% faster gun reload, 20% faster rudder shift, 20% faster acceleration, 5% better stealth, 10% longer Smoke duration, 10% more speed, 2% better chance of fire, 5% better chance of causing Flooding, 5% more Torp Damage.
However, each Grind allows me to buy 1 item from the list. Suddenly, this becomes a VERY serious choice. I have to look at the ship I want to upgrade, how I play it, how it works in the Meta, try to guess at future meta changes, what the focus of the ship will be. Because it's not like I can switch them out later. No, this is a permenant reward upgrade for that ship. If I want another reward, I have to re-grind the line yet again and I can only do it 3 times.
So, my choice for the Khaba would be based on Speed, Guns and HP. Or maybe Rudder shift? Or acceleration? It's a hard choice! It also means, if the Khaba I am facing goes a different path to what I would choose, it is so much nastier as you never quite know what to expect.
For my Shimakaze, Rudder Shift is nice, speed is useful, torp reload is a must, but do I go for more damage? Better smoke? etc.
This, to me, seems a far better idea that having a fixed upgrade applied across the board. Why give the same bonuses to the Kremlin, a ship based around getting in close and brawling, that you do to the Yamato? Why give the same bonuses to the Khaba that you do to the Shimakaze? Let alone the Moskva Vs the Des Moines vs Henri IV. Different lines have utterly different play styles, despite being in the same 'Class' of ship. You may want to re-consider giving them the same bonuses. However, having a pool to buy from, that exceeds the total amount you could purchase, means more variety of play.
10. Speed. I took 2 years to reach where I am, however, I have a Premium account (Thank you delicious Christmas Loot Boxes in 2017) and am prepared to shell out money for Dubloons so I can re-use Modules, convert Free XP etc. I'm not typical in that regard I'd imagine. However, I have seen Flamu and other CC's quite literally grind from T1 through to T10 in a matter of hours. Because they are that good. Sure, they have to do it in a maximum of 3 lines, once per season, so one week of re-grinding per season and they remain the best players in the game, but with the best ships now. Me? I'm looking at many months because I am just not that good. Suddenly, I may have one line re-ground, but 2 more are untouched and the next season has rocked around, leaving me falling further behind the players who are pushing for maximum power, simply because I don't have the time to play as much. This then prompts WarGamming to sell 'grind boosters' or even just the Premium Economy Flags,which means I need something else to keep up on.
So, yeah. That was a bit longer than I planned.
TL;DR I like the idea. I really really like the idea, but it's going to need some serious re-thought from what was indicated to make it viable for the game long term. Otherwise, it's going to cause players to walk away because they can't keep up.
On the other hand, they may also find they just focus on one ship entirely and leave it at that. Your Methods may vary.