Legacy (adj.) means “inherited”.¹ Legacy systems, for example, are the systems we inherited. In the software industry, it often has a negative connotation that the thing is obsolete (but not always²). An antonym would refer to recent innovations, so for that we would use terms such as:
- cutting-edge systems
- the latest systems
- mainstream systems
- modern systems
- present-day systems
- state-of-the-art systems
- trendy systems³
Each of these has a unique shade of meaning and you would simply pick the one that is closest in meaning to what you intend.
¹ “legacy”, Collins Free Online Dictionary. This entry includes several examples of legacy as a noun modifier. These examples show that legacy always connotes inheritance, but the connotation of obsolescence comes from context. For instance, a legacy moon is apparently an astrological phenomenon:
high legacy software support costs —Computing (2010)
the hope of securing legacy gifts —The Sun (2016)
something for the legacy committee to consider —Times, Sunday Times (2012)
still using army legacy computer systems —Times, Sunday Times (2013)
a legacy moon suggests something —The Sun (2010)
² Stanley Quayle, “Ticketmaster and its ‘old’ system”, VMS Spoken Here. Sometimes, according to Mr Quayle, “legacy means stuff that just works”.
³ Italicized terms collected from the entry “cutting edge”, Thesaurus.com.