Late on this topic as I picked up the game on sale recently, but I'll add my 2 cents.

Personally I think the current match engine plays well, and I wouldn't change it much. I know this is a contentious opinion, but I think it has a good balance between simulation and arcade-like feel in the current state.

Aside from the obvious licensing headache that Rugby games always have, I'd like to see more focus put into fleshing out game modes and leagues instead of the endless tweaking to the match engine.

Career mode wasn't horrible, but it would be nice to see it detached from the booster pack system and gain more management sim components. Split it into Coach and Player careers even (maybe Team if you want to introduce a brand new team to a league).

Coach career mode:
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Coach career mode could see you get coaching offers from a few minor teams and then either build the team up or take offers for new teams.

Enforce elements like national quotas and make finance trickier to manage, to avoid or slow down the loaded star player situation.

Have dynamic player stats that change based on their performance during the seasons, like confidence and form meters etc.

Always run full Rugby calendars in any career mode, and have players drafted for national teams etc. (something like the PRM career mode). So as a club coach you lose players during national season, or as a national coach you draft players from the current clubs based on their performance and dynamic stats etc.

Booster/Lootbox/Online league:
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I like the model that EA and 2K have adopted, where their lootbox mode is isolated and doesn't really seep into the rest of the game. Give the player a "supercard" team with it's own online/offline progression lane. Have single player challenges or leagues that score points to buy booster packs, or online leagues that you can sign your team up for.

This can be the micro-transaction zone to keep the game funded and scratch that itch for players who enjoy this, or be a booster-driven offline team for those who like to these systems. I've found it worked really well in games like WWE and UFC, and feels segregated enough if you don't like it.