Technically modern legal, but this is very much a dumb casual deck. It has potential if a good 3 cost version of the ability gets printed, say a 1 mana 1/1 with 2R: deal 2. If you want to build a competitive version of this deck, just build U/R Storm.
That said it does have early combo potential and can blow people out with a bunch of nothing cards if your opponent isn't expecting it.
Built around the two red Invoker's (Flamewave Invoker and Valakut Invoker) as part of a challenge. Don't worry, I know that there's much more efficient and powerful thing's to do with this kind of mana.
On the upside, Training Grounds can be stacked, reducing your Invoker costs to 6(1), 4(2), 2(3), 1(4).
The manabase is just the best lands I could come up with. Realistically if you're putting this together, you're just gonna use budget lands like painlands and basics. Utopia Sprawl is pretty terrible, but I could find a modern legal creature that could net you mana off an activated ability when used with Training Grounds
The main interactions:
Rings of Brighthearth and an Invoker = 10 mana for 10 damage to a player or two lighting bolts. This is why we run an infinite mana combo. They're bad. Bad bad cards. I love them though.
Training Grounds, Izzet Guildmage and Manamorphose = Infinite Mana and Draws. Requires 3 mana to start. Can be done turn three. This is actually the best part of the deck. If you get it off, you win. Technically the mana isn't infinite as you only make 1 a cycle and can only combo at most 52 times (Manamorphose forces you to draw the card), but then you'll have all 4 soulbright who make 3 mana net each (12 total) in this scenario upping you to 64 mana, use 3 to cast your other 3 training grounds (61), 3 to cast an Invoker (58) and then you can invoke 58 times (290 damage with Flamewave, 174 with Valakut).
Training Grounds and Soulbright Flamekin = 8 Red mana for 3 mana of any colour. Can be done turn three. Allows you to surprise push through the previous combo as it can be done with all red mana
Training Grounds and Magus of the Candelabra = Untap 3 lands for 1. Can be done turn two (though inefficient obviously.)
Training Grounds, Rings of Brighthearth and Magus of the Candelabra = Untap 6 lands for 3. Yes the lands can be tapped for mana in between resolving the two untap effects.
