Reduce bots, spam.
Make sure that verified users are not able to change their usernames anyhow they want (currently you will find hundreds of elon musk verified accounts).
Improve twitter mobile app (less bloat unlike reddit app)
Use crypto payments to pay for twitter blue. Crypto payments meaning not only BTC and Dogecoin but privacy coins like Monero, Dash, Zcash etc.
Have Twitter Blue Pro, so that users will get no ads (currently twitter blue gets half ads), first access to new features, something like substack articles where twitter blue pro users will be able to read blogs created by twitter users (tiwtter is a 'micro-blogging' platform afterall) etc.
Bring new features which are aggregated from other apps like (this point is not that good)
a.) Unlimited video upload- youtube
b.) Stories-Instagram
c.) Short videos- tiktok
d.) Threads-Reddit etc
Reduce excessive tracking of users on its site and app.
Try to attract non-english, non-western users. Most non-english audience does not even know/care about twitter. They are all on whatsapp, youtube, tiktok etc.
Have NSFW, gambling and generally immoral stuff promotion to earn money on the sidelines. (The NSFW stuff will be opt-in by default to make sure that twitter is 'family friendly)
P2P transfer of fund from users to content creators on twitter itself like Patreon, Brave BAT twitter tipping etc. Twitter will get like 5% cut from the tips.
Have alternative payment methods like UPI (india), SEPA (europe) and not only card payment via stripe.