Personally I loved every gametype i played; i have a few criticisms but i'll keep them brief:
Oddball - the best minigame in halo just got better. love the way the new flaming skull looks, just throwing that out there.
Covy Slayer - There is a significant difference between all-spartan slayer and the new all-elites slayer. as spartan armor is brightly colored red or blue, its easy to immediately tell friend from foe. the covenant counterpart features both teams in darker shades; covy blue is almost purple, and covy red is (also) almost purple. the net effect is that many times in game, i would walk right by an opponent and not notice, mistaking him for a teammate.. or vice versa.
Stockpile - sometimes this game would start with teams that were uneven, i.e. 4 vs 2. as a player cant be in two places at once, the outnumbered players lost every time no matter what; it takes two hands to carry a flag, and the team with more hands wins, hands-down (pun intended).
Invasion- Excellent idea, and i had a thought possibly that could make both gametypes (invasion and invasion slayer)even more epic.. NPC allies. in Invasion, each invasion phase could spawn waves of certain npc types to assist their faction's players, i.e., UNSC maries might come pouring out of the base after the shield generators go down, or a dropship might deliver a pack of grunts and a hunter pair to assist with the covenant extraction. in Invasion: slayer, each captured destination might spawn reinforcements in the form of allied npcs to the capturing team; killing them wouldnt net points, but not killing them could result in getting overwhelmed. it would make capturing the destinations a more valuable objective, as the side that consistently captures territory begins to amass quite an army as the "invasion" itself begins to escalate.
Sidenote: Bring Back Griffball! lol