1.Stats: Should Walrein get a stat boost, Evolution, Mega Evolution or nothing?
Walrein already has 530 BST, so any update would be +10 more BST. Walrein is also obviously incapable of evolving further.
This leaves Mega Evolution as a possible broker for a proper min-maxing of stats, as the only stat Walrein has that is over 100 is its HP, and it is unlikely that Walrein would get a 10 more stat for Special Attack when its physical repertoire is larger.
2.Should Walrein get more abilities?
All the abilities Walrein has are not too good. Ice Body was infamous for StallRein sets, but those haven't been relevant since ages ago. Strong Jaw and Water Veil are the only obvious ones that seem to be present, though.
3. Should Walrein get more moves?
Yes. Most obvious missing moves for Walrein are a recovery move like Slack Off, a Special offense boosting move like Calm Mind and Nasty Plot, and a wider range of coverage moves like Play Rough, Zen Headbutt, Hyper Voice, Stone Edge, and Belch.
4.What additional typing Walrein can have?
Normal, Dark and Rock are the most obvious ones, followed by Ground and Steel.