Undergrowth and Mentor seem simple enough (Counting graveyard creatures is easy for a card, as is counting power on attackers, although the ordering issue will still be in effect).
Convoke will probably just be “Costs X less to cast where X is the number of creatures and creature reinforcements you control” but (hopefully) with some extra condition so everything doesn’t become free with some tokens.
Surveil is easy, there are already a few cards that do that
Jump Start is the one I’m not sure about. I guess it could be done with an activate gem (like aftermath was). Probably don’t even need the discard.
I’m most concerned with how they’ll implement Split cards.
The easiest way to do those would be to jury rig the ‘disable cards in hand’ mechanic in a similar way to how they implemented Flash: if the split card is Disabled, then side B of the card will cast when it’s full of mana, but if it’s Enabled then side A will cast.
I really hope they don’t do this, because then you won’t actually be able to disable the card so it doesn’t cast until you want it to.
I think they could just modify the transform mechanic and let you choose which side of the card you want to cast. Or give you a prompt when you cast it that asks which side you want to cast .
As for the new mechanics I’m most concerned about jump start. It’ll probably work similarly to aftermath. I just don’t know how I feel about that.
The spent spell could create activated gems (like aftermath spells) and then when you match one, you discard a card and are drained of mana, then the spell casts again.
Convoke maybe could work that the spells with convoke either gain mana when a creature enters, like crew, or when a creature attacks, like Radha, though I think the better way to do it would be to disable your creatures in exchange for mana, though exactly how you’d do that, I don’t know.
I just really hope that they finally give us the option to at least know how many cards are in our graveyards at this point, though ideally I’d like to see what they are, especially with how the new Liliana works. You dump cards in your graveyard and never get to see what went there.
Jump Start - Will probably be similar to Aftermath.
Undergrowth - Would be pretty easy considering we have Lord of Extinction in game. Although he doesn’t just count one card type. This may be the mechanic to push for knowing the graveyard.
Mentor - Would be probably a mirror to paper, but it may be slightly different.
Convoke - I can see it being like Crew. Then again it could take on a different twist such as a cost reduction effect. “Cost X less for each creature and reinforcement.” X could be a low number. Like a 9 mana creature only get 1 for each. Then again if it didn’t include reinforcements then a 9 mana creature could possibly get 2 for each. 9 mana with 0 creatures, 7 with one, 5 with two, and 3 with three. And if that 9 mana creature was like a 2/2 or 2/3 with no other abilities it costing 3 mana if you had three creatures wouldn’t be too powerful.