I was planning to write a lot on how well the WoT gold system works but then Tokomak got me 
But yes, I agree it is a very nice f2p system that feels “right” and fair, in most aspects. I started playing WoT about 1.5 year ago, with several longer breaks, and think I’ve spent about 3 x 20 Euro on it so far. I’ve used some gold for buying more garage slots (as WoT have a certain “collector” aspect, so you want to keep some tanks that you really enjoyed, forever), but mostly for a month of premium account, for those periods where I plan to play a lot. The premium account is just a booster for ingame xp and credits, no payforwin at all. I really like this freedom - even if you get a bit addicted to the “extra” premium income, it’s not a problem to play without it from time to time.
WoT is a bit different than normal shooters so their ideas can not be adopted directly, but I’ve been thinking on how some of it could be used in DB, and will just toss out a few ideas - some of it might have been suggested already, if so forgive me:
Weapon depot: Let’s say you do it a bit like in Brink, and add tons of pretty balanced weapons to the game. For each weapon people have to first research them with in-game xp, then they can purchase them for in-game credits. But by default you will only have a couple of storage slots for each weapon class (like the 2 loadouts we have today), so there is a limit on how many weapons you can keep available in your depot. If your depot is full, you have to sell one gun to buy another from the same class. Then you can buy more storage slots for gold, to have more freedom in choice. It is ofc important that no guns are much better than others of the same class, but I think you managed that pretty well with the last Brink update.
This could perhaps also be splitted into 2 different types of slots - one set for your total collection, which are the those you can pick from before the match. While another type could be those weapons you can “carry” with you into a match, and then change during a respawn.
I would limit this to guns only, as other types of special equipment and boosters should rather be campaign rewards, like the in ET and ETQW - temporary upgrades pr. campaign, play2win hehe. I really love the campaign progress system you have in those games, and hope it will be back in DB as well 
Another pay for gold thing could be extra character slots, where you can setup pre-defined loadouts so they’re easily available before a match. For free you get 3 slots for example, then you have to pay gold for extra.
You could perhaps also adopt the in-game credits system, where you have to pay for your ammo but get credits back for dealing damage. Or just skip the ammo cost (it works good in WoT due to the superlow RoF with expensive shells but it would probably hurt team work in DB) but the credit system itself is good - and credit income is based on the amount of damage you deal to enemies, not kills.
Well that was maybe not a lot of ideas or very deeply thought out, but maybe some of it could be interesting.
Just remember it’s really really important to not get the pay2win stamp on the game - avoid such things at all cost!
It’s also important that players doesn’t feel too much pressure to pay - better with carrot than stick. Some clever marketing and special events can do the trick.
The CEO of WoT wrote a really good article on their business model some months ago, and I really liked their thinking. They said that even if only 1 out of 3 players (don’t remember the ratio exactly but something like that) actually paid anything at all, they still gave full support to everyone, paying or not - because it’s better with a long-term player base that stays, than a short-term boost of income and then it’s all over. It’s a delicate balance, but I really hope (and think you will) find it 
)